Value of the last installment.












0












$begingroup$


The price of a TV set is ₹20,000 to be paid in 20 installments of ₹1,000 each. Rate of interest=6% p.a



The first installment is to be paid at the time of purchase, then what will be the value of the last installment covering the interest as well?



There are two explanations I found:




  1. Money paid in cash = ₹ 1000
    Balance payment = (20000 - 1000) = ₹ 19000
    (the above is from R.S. Aggarwal book and doesn't seem complete)

  2. These links explain and conclude the answer to be 1950 but I didn't exactly understand: link 1 and link 2


I thought something like below (assuming monthly installments, hence taking 6/12 as rate of interest):




  • Value after $20$ installments $= 20000 + (20000cdot 20cdot 6)/(12cdot 100) = 22000$

  • So, value after 19 installments $=
    1000+(1000cdot 20cdot 6)/(12cdot 100) +....+ 1000+(1000cdot 2cdot 6)/(12cdot 100)
    = 19000 + (1000cdot 6/(12cdot 100))cdot(19cdot 20/2) = 19950$


  • Thus, value of last installment $= 22000-19950=2050$


Can someone explain what am I doing wrong?










share|cite|improve this question











$endgroup$

















    0












    $begingroup$


    The price of a TV set is ₹20,000 to be paid in 20 installments of ₹1,000 each. Rate of interest=6% p.a



    The first installment is to be paid at the time of purchase, then what will be the value of the last installment covering the interest as well?



    There are two explanations I found:




    1. Money paid in cash = ₹ 1000
      Balance payment = (20000 - 1000) = ₹ 19000
      (the above is from R.S. Aggarwal book and doesn't seem complete)

    2. These links explain and conclude the answer to be 1950 but I didn't exactly understand: link 1 and link 2


    I thought something like below (assuming monthly installments, hence taking 6/12 as rate of interest):




    • Value after $20$ installments $= 20000 + (20000cdot 20cdot 6)/(12cdot 100) = 22000$

    • So, value after 19 installments $=
      1000+(1000cdot 20cdot 6)/(12cdot 100) +....+ 1000+(1000cdot 2cdot 6)/(12cdot 100)
      = 19000 + (1000cdot 6/(12cdot 100))cdot(19cdot 20/2) = 19950$


    • Thus, value of last installment $= 22000-19950=2050$


    Can someone explain what am I doing wrong?










    share|cite|improve this question











    $endgroup$















      0












      0








      0





      $begingroup$


      The price of a TV set is ₹20,000 to be paid in 20 installments of ₹1,000 each. Rate of interest=6% p.a



      The first installment is to be paid at the time of purchase, then what will be the value of the last installment covering the interest as well?



      There are two explanations I found:




      1. Money paid in cash = ₹ 1000
        Balance payment = (20000 - 1000) = ₹ 19000
        (the above is from R.S. Aggarwal book and doesn't seem complete)

      2. These links explain and conclude the answer to be 1950 but I didn't exactly understand: link 1 and link 2


      I thought something like below (assuming monthly installments, hence taking 6/12 as rate of interest):




      • Value after $20$ installments $= 20000 + (20000cdot 20cdot 6)/(12cdot 100) = 22000$

      • So, value after 19 installments $=
        1000+(1000cdot 20cdot 6)/(12cdot 100) +....+ 1000+(1000cdot 2cdot 6)/(12cdot 100)
        = 19000 + (1000cdot 6/(12cdot 100))cdot(19cdot 20/2) = 19950$


      • Thus, value of last installment $= 22000-19950=2050$


      Can someone explain what am I doing wrong?










      share|cite|improve this question











      $endgroup$




      The price of a TV set is ₹20,000 to be paid in 20 installments of ₹1,000 each. Rate of interest=6% p.a



      The first installment is to be paid at the time of purchase, then what will be the value of the last installment covering the interest as well?



      There are two explanations I found:




      1. Money paid in cash = ₹ 1000
        Balance payment = (20000 - 1000) = ₹ 19000
        (the above is from R.S. Aggarwal book and doesn't seem complete)

      2. These links explain and conclude the answer to be 1950 but I didn't exactly understand: link 1 and link 2


      I thought something like below (assuming monthly installments, hence taking 6/12 as rate of interest):




      • Value after $20$ installments $= 20000 + (20000cdot 20cdot 6)/(12cdot 100) = 22000$

      • So, value after 19 installments $=
        1000+(1000cdot 20cdot 6)/(12cdot 100) +....+ 1000+(1000cdot 2cdot 6)/(12cdot 100)
        = 19000 + (1000cdot 6/(12cdot 100))cdot(19cdot 20/2) = 19950$


      • Thus, value of last installment $= 22000-19950=2050$


      Can someone explain what am I doing wrong?







      finance






      share|cite|improve this question















      share|cite|improve this question













      share|cite|improve this question




      share|cite|improve this question








      edited Jan 21 at 20:54









      alexjo

      12.5k1430




      12.5k1430










      asked Jan 20 at 11:13









      ANANYA AKHOURIANANYA AKHOURI

      81




      81






















          1 Answer
          1






          active

          oldest

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          0












          $begingroup$

          Let be $P=1,000$ the installment, $n=20$ number of monthly installments, $S=20,000$ the value of the TV.



          Observing that $Ptimes n=1,000times 20=20,000=S$, we have that installments are for the principal amount $S$ only and interest is yet to be paid.



          The interest $I$ is to be paid with the amount of last installment $L$ and hence last installment will be
          $$
          L=P+I
          $$

          Let be $i=6%$ the annual interest rate. the duration of each month is $1/12$ of a year. With the simple interest we have
          $$
          I=text{principal}times text{interest rate}times text{time}
          $$

          At month $1$, the principal is $S_1=S-P=19,000$ and the interest to be paid at month $20$ is $$I_1=S_1times itimes 1=19,000times0.06times 1/12$$



          At month $2$, the principal is $S_2=S_1-P=18,000$ and the interest to be paid at month $20$ is $$I_2=S_2times itimes 1=18,000times0.06times 1/12$$



          At month $19$, the principal is $S_{19}=S_{18}-P=1,000$ and the interest $$I_{19}=S_{19}times itimes 1=1,000times0.06times 1/12$$



          At month $20$, the principal is $S_{20}=S_{19}-P=0$ and the interest is zero.



          Summing up all the interests we have
          $$
          I=sum_{k=1}^{20}I_k=sum_{k=1}^{19}I_k=sum_{k=1}^{19} S_{k}times itimes 1/12
          $$

          Observing $S_k=1000times k$ we have
          $$
          I=frac{1}{12}sum_{k=1}^{19} S_{k}times i=frac{1}{12}sum_{k=1}^{19} 1000ktimes i=frac{1000times 0.06}{12}sum_{k=1}^{19} k=5sum_{k=1}^{19} k=5frac{19times 18}{2}=950
          $$

          beacause $sum_{k=1}^n k=frac{n(n-1)}{2}$.



          so we have that the last installmenet is
          $$
          L=P+I=1,000+950=1,950
          $$






          share|cite|improve this answer









          $endgroup$













          • $begingroup$
            I understood! The interest is being calculated on the principal that is left. Thanks a ton! :) However, one thing that still confuses me is that when we paid our first installment of Rs. 1000 at the time of purchase, why wasn't interest calculated on the whole amount of 20,000? That is 2000*0.06*1/12=100. Why wasn't it added to the last installment?
            $endgroup$
            – ANANYA AKHOURI
            Jan 22 at 15:14










          • $begingroup$
            Because you payed 1000 immediately and you left 19000 on which you have to pay interest. Equivalently you pay on 20, 0000 the interest $I_0=20,000times 6% times 0/12=0$.
            $endgroup$
            – alexjo
            Jan 22 at 15:19










          • $begingroup$
            Oh..understood now! Thanks a lot!! :)
            $endgroup$
            – ANANYA AKHOURI
            Jan 22 at 18:35











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          0












          $begingroup$

          Let be $P=1,000$ the installment, $n=20$ number of monthly installments, $S=20,000$ the value of the TV.



          Observing that $Ptimes n=1,000times 20=20,000=S$, we have that installments are for the principal amount $S$ only and interest is yet to be paid.



          The interest $I$ is to be paid with the amount of last installment $L$ and hence last installment will be
          $$
          L=P+I
          $$

          Let be $i=6%$ the annual interest rate. the duration of each month is $1/12$ of a year. With the simple interest we have
          $$
          I=text{principal}times text{interest rate}times text{time}
          $$

          At month $1$, the principal is $S_1=S-P=19,000$ and the interest to be paid at month $20$ is $$I_1=S_1times itimes 1=19,000times0.06times 1/12$$



          At month $2$, the principal is $S_2=S_1-P=18,000$ and the interest to be paid at month $20$ is $$I_2=S_2times itimes 1=18,000times0.06times 1/12$$



          At month $19$, the principal is $S_{19}=S_{18}-P=1,000$ and the interest $$I_{19}=S_{19}times itimes 1=1,000times0.06times 1/12$$



          At month $20$, the principal is $S_{20}=S_{19}-P=0$ and the interest is zero.



          Summing up all the interests we have
          $$
          I=sum_{k=1}^{20}I_k=sum_{k=1}^{19}I_k=sum_{k=1}^{19} S_{k}times itimes 1/12
          $$

          Observing $S_k=1000times k$ we have
          $$
          I=frac{1}{12}sum_{k=1}^{19} S_{k}times i=frac{1}{12}sum_{k=1}^{19} 1000ktimes i=frac{1000times 0.06}{12}sum_{k=1}^{19} k=5sum_{k=1}^{19} k=5frac{19times 18}{2}=950
          $$

          beacause $sum_{k=1}^n k=frac{n(n-1)}{2}$.



          so we have that the last installmenet is
          $$
          L=P+I=1,000+950=1,950
          $$






          share|cite|improve this answer









          $endgroup$













          • $begingroup$
            I understood! The interest is being calculated on the principal that is left. Thanks a ton! :) However, one thing that still confuses me is that when we paid our first installment of Rs. 1000 at the time of purchase, why wasn't interest calculated on the whole amount of 20,000? That is 2000*0.06*1/12=100. Why wasn't it added to the last installment?
            $endgroup$
            – ANANYA AKHOURI
            Jan 22 at 15:14










          • $begingroup$
            Because you payed 1000 immediately and you left 19000 on which you have to pay interest. Equivalently you pay on 20, 0000 the interest $I_0=20,000times 6% times 0/12=0$.
            $endgroup$
            – alexjo
            Jan 22 at 15:19










          • $begingroup$
            Oh..understood now! Thanks a lot!! :)
            $endgroup$
            – ANANYA AKHOURI
            Jan 22 at 18:35
















          0












          $begingroup$

          Let be $P=1,000$ the installment, $n=20$ number of monthly installments, $S=20,000$ the value of the TV.



          Observing that $Ptimes n=1,000times 20=20,000=S$, we have that installments are for the principal amount $S$ only and interest is yet to be paid.



          The interest $I$ is to be paid with the amount of last installment $L$ and hence last installment will be
          $$
          L=P+I
          $$

          Let be $i=6%$ the annual interest rate. the duration of each month is $1/12$ of a year. With the simple interest we have
          $$
          I=text{principal}times text{interest rate}times text{time}
          $$

          At month $1$, the principal is $S_1=S-P=19,000$ and the interest to be paid at month $20$ is $$I_1=S_1times itimes 1=19,000times0.06times 1/12$$



          At month $2$, the principal is $S_2=S_1-P=18,000$ and the interest to be paid at month $20$ is $$I_2=S_2times itimes 1=18,000times0.06times 1/12$$



          At month $19$, the principal is $S_{19}=S_{18}-P=1,000$ and the interest $$I_{19}=S_{19}times itimes 1=1,000times0.06times 1/12$$



          At month $20$, the principal is $S_{20}=S_{19}-P=0$ and the interest is zero.



          Summing up all the interests we have
          $$
          I=sum_{k=1}^{20}I_k=sum_{k=1}^{19}I_k=sum_{k=1}^{19} S_{k}times itimes 1/12
          $$

          Observing $S_k=1000times k$ we have
          $$
          I=frac{1}{12}sum_{k=1}^{19} S_{k}times i=frac{1}{12}sum_{k=1}^{19} 1000ktimes i=frac{1000times 0.06}{12}sum_{k=1}^{19} k=5sum_{k=1}^{19} k=5frac{19times 18}{2}=950
          $$

          beacause $sum_{k=1}^n k=frac{n(n-1)}{2}$.



          so we have that the last installmenet is
          $$
          L=P+I=1,000+950=1,950
          $$






          share|cite|improve this answer









          $endgroup$













          • $begingroup$
            I understood! The interest is being calculated on the principal that is left. Thanks a ton! :) However, one thing that still confuses me is that when we paid our first installment of Rs. 1000 at the time of purchase, why wasn't interest calculated on the whole amount of 20,000? That is 2000*0.06*1/12=100. Why wasn't it added to the last installment?
            $endgroup$
            – ANANYA AKHOURI
            Jan 22 at 15:14










          • $begingroup$
            Because you payed 1000 immediately and you left 19000 on which you have to pay interest. Equivalently you pay on 20, 0000 the interest $I_0=20,000times 6% times 0/12=0$.
            $endgroup$
            – alexjo
            Jan 22 at 15:19










          • $begingroup$
            Oh..understood now! Thanks a lot!! :)
            $endgroup$
            – ANANYA AKHOURI
            Jan 22 at 18:35














          0












          0








          0





          $begingroup$

          Let be $P=1,000$ the installment, $n=20$ number of monthly installments, $S=20,000$ the value of the TV.



          Observing that $Ptimes n=1,000times 20=20,000=S$, we have that installments are for the principal amount $S$ only and interest is yet to be paid.



          The interest $I$ is to be paid with the amount of last installment $L$ and hence last installment will be
          $$
          L=P+I
          $$

          Let be $i=6%$ the annual interest rate. the duration of each month is $1/12$ of a year. With the simple interest we have
          $$
          I=text{principal}times text{interest rate}times text{time}
          $$

          At month $1$, the principal is $S_1=S-P=19,000$ and the interest to be paid at month $20$ is $$I_1=S_1times itimes 1=19,000times0.06times 1/12$$



          At month $2$, the principal is $S_2=S_1-P=18,000$ and the interest to be paid at month $20$ is $$I_2=S_2times itimes 1=18,000times0.06times 1/12$$



          At month $19$, the principal is $S_{19}=S_{18}-P=1,000$ and the interest $$I_{19}=S_{19}times itimes 1=1,000times0.06times 1/12$$



          At month $20$, the principal is $S_{20}=S_{19}-P=0$ and the interest is zero.



          Summing up all the interests we have
          $$
          I=sum_{k=1}^{20}I_k=sum_{k=1}^{19}I_k=sum_{k=1}^{19} S_{k}times itimes 1/12
          $$

          Observing $S_k=1000times k$ we have
          $$
          I=frac{1}{12}sum_{k=1}^{19} S_{k}times i=frac{1}{12}sum_{k=1}^{19} 1000ktimes i=frac{1000times 0.06}{12}sum_{k=1}^{19} k=5sum_{k=1}^{19} k=5frac{19times 18}{2}=950
          $$

          beacause $sum_{k=1}^n k=frac{n(n-1)}{2}$.



          so we have that the last installmenet is
          $$
          L=P+I=1,000+950=1,950
          $$






          share|cite|improve this answer









          $endgroup$



          Let be $P=1,000$ the installment, $n=20$ number of monthly installments, $S=20,000$ the value of the TV.



          Observing that $Ptimes n=1,000times 20=20,000=S$, we have that installments are for the principal amount $S$ only and interest is yet to be paid.



          The interest $I$ is to be paid with the amount of last installment $L$ and hence last installment will be
          $$
          L=P+I
          $$

          Let be $i=6%$ the annual interest rate. the duration of each month is $1/12$ of a year. With the simple interest we have
          $$
          I=text{principal}times text{interest rate}times text{time}
          $$

          At month $1$, the principal is $S_1=S-P=19,000$ and the interest to be paid at month $20$ is $$I_1=S_1times itimes 1=19,000times0.06times 1/12$$



          At month $2$, the principal is $S_2=S_1-P=18,000$ and the interest to be paid at month $20$ is $$I_2=S_2times itimes 1=18,000times0.06times 1/12$$



          At month $19$, the principal is $S_{19}=S_{18}-P=1,000$ and the interest $$I_{19}=S_{19}times itimes 1=1,000times0.06times 1/12$$



          At month $20$, the principal is $S_{20}=S_{19}-P=0$ and the interest is zero.



          Summing up all the interests we have
          $$
          I=sum_{k=1}^{20}I_k=sum_{k=1}^{19}I_k=sum_{k=1}^{19} S_{k}times itimes 1/12
          $$

          Observing $S_k=1000times k$ we have
          $$
          I=frac{1}{12}sum_{k=1}^{19} S_{k}times i=frac{1}{12}sum_{k=1}^{19} 1000ktimes i=frac{1000times 0.06}{12}sum_{k=1}^{19} k=5sum_{k=1}^{19} k=5frac{19times 18}{2}=950
          $$

          beacause $sum_{k=1}^n k=frac{n(n-1)}{2}$.



          so we have that the last installmenet is
          $$
          L=P+I=1,000+950=1,950
          $$







          share|cite|improve this answer












          share|cite|improve this answer



          share|cite|improve this answer










          answered Jan 21 at 20:54









          alexjoalexjo

          12.5k1430




          12.5k1430












          • $begingroup$
            I understood! The interest is being calculated on the principal that is left. Thanks a ton! :) However, one thing that still confuses me is that when we paid our first installment of Rs. 1000 at the time of purchase, why wasn't interest calculated on the whole amount of 20,000? That is 2000*0.06*1/12=100. Why wasn't it added to the last installment?
            $endgroup$
            – ANANYA AKHOURI
            Jan 22 at 15:14










          • $begingroup$
            Because you payed 1000 immediately and you left 19000 on which you have to pay interest. Equivalently you pay on 20, 0000 the interest $I_0=20,000times 6% times 0/12=0$.
            $endgroup$
            – alexjo
            Jan 22 at 15:19










          • $begingroup$
            Oh..understood now! Thanks a lot!! :)
            $endgroup$
            – ANANYA AKHOURI
            Jan 22 at 18:35


















          • $begingroup$
            I understood! The interest is being calculated on the principal that is left. Thanks a ton! :) However, one thing that still confuses me is that when we paid our first installment of Rs. 1000 at the time of purchase, why wasn't interest calculated on the whole amount of 20,000? That is 2000*0.06*1/12=100. Why wasn't it added to the last installment?
            $endgroup$
            – ANANYA AKHOURI
            Jan 22 at 15:14










          • $begingroup$
            Because you payed 1000 immediately and you left 19000 on which you have to pay interest. Equivalently you pay on 20, 0000 the interest $I_0=20,000times 6% times 0/12=0$.
            $endgroup$
            – alexjo
            Jan 22 at 15:19










          • $begingroup$
            Oh..understood now! Thanks a lot!! :)
            $endgroup$
            – ANANYA AKHOURI
            Jan 22 at 18:35
















          $begingroup$
          I understood! The interest is being calculated on the principal that is left. Thanks a ton! :) However, one thing that still confuses me is that when we paid our first installment of Rs. 1000 at the time of purchase, why wasn't interest calculated on the whole amount of 20,000? That is 2000*0.06*1/12=100. Why wasn't it added to the last installment?
          $endgroup$
          – ANANYA AKHOURI
          Jan 22 at 15:14




          $begingroup$
          I understood! The interest is being calculated on the principal that is left. Thanks a ton! :) However, one thing that still confuses me is that when we paid our first installment of Rs. 1000 at the time of purchase, why wasn't interest calculated on the whole amount of 20,000? That is 2000*0.06*1/12=100. Why wasn't it added to the last installment?
          $endgroup$
          – ANANYA AKHOURI
          Jan 22 at 15:14












          $begingroup$
          Because you payed 1000 immediately and you left 19000 on which you have to pay interest. Equivalently you pay on 20, 0000 the interest $I_0=20,000times 6% times 0/12=0$.
          $endgroup$
          – alexjo
          Jan 22 at 15:19




          $begingroup$
          Because you payed 1000 immediately and you left 19000 on which you have to pay interest. Equivalently you pay on 20, 0000 the interest $I_0=20,000times 6% times 0/12=0$.
          $endgroup$
          – alexjo
          Jan 22 at 15:19












          $begingroup$
          Oh..understood now! Thanks a lot!! :)
          $endgroup$
          – ANANYA AKHOURI
          Jan 22 at 18:35




          $begingroup$
          Oh..understood now! Thanks a lot!! :)
          $endgroup$
          – ANANYA AKHOURI
          Jan 22 at 18:35


















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