SOLUTION: Suppose $726.56 is deposited at the end of every six months into an account earning 6.45% compounded semi-annually. If the balance in the account four years after the last deposit

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Question 1207543: Suppose $726.56 is deposited at the end of every six months into an account earning 6.45% compounded semi-annually. If the balance in the account four years after the last deposit is to be $31 300.00, how many deposits are needed?
Answer by ikleyn(52915) About Me  (Show Source):
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Suppose $726.56 is deposited at the end of every six months into an account earning 6.45%
compounded semi-annually. If the balance in the account four years after the last deposit
is to be $31 300.00, how many deposits are needed?
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Use the formula for an Ordinary Annuity saving account compounded semi-annually 


    FV = P%2A%28%28%281%2Br%2F2%29%5En-1%29%2F%28%28r%2F2%29%29%29


where FV is the future value, P is the payment at the end of every 6 months;
r is the interest rate per year expressed as decimal, 
n is the number of semi-annual deposits.


So, we need to find " n " from this equation


    %28%281%2B0.0645%2F2%29%5En-1%29%2F%28%280.0645%2F2%29%29 = FV%2FP = 31300%2F726.56,  which is the same as

    %28%281%2B0.0645%2F2%29%5En-1%29%2F%280.0645%2F2%29 = 43.07971812, 

    %281%2B0.0645%2F2%29%5En-1 = %280.0645%2F2%29%2A43.07971812,

    %281%2B0.0645%2F2%29%5En-1 = 1.389320909,

    %281%2B0.0645%2F2%29%5En = 1+1.389320909.

    %281%2B0.0645%2F2%29%5En = 2.389320909.


Take logarithm base 10 of both sides

    n*log(1+0.0645/12) = log(2.389320909)


and calculate  

     n = log%28%282.389320909%29%29%2Flog%28%281%2B0.0645%2F2%29%29 = 27.44 time periods.


Round it to the closest greater integer, which is 28 semi-annual periods,
in order for the bank would be in position to compete the last compounding.

28 semi-annual periods is the same as 14 years.


ANSWER.  14 years.

Solved.