Question 932105
<pre>
Let  r = interest rate
     n = number of intervals
     t = duration of the payment
     A = monthly installment 
    PV = Present Value
    FV = Final Value

1.
{{{FV = PV(1+r/n)^(nt-1)+a(((1+r/n)^(nt)-1)/(r/n))}}}
{{{FV = 4000(1+.05/12)^((12)(3)-1)+100(((1+.05/12)^((12)(3))-1)/(.05/12))}}}
{{{FV = 4000(1.004166667)^(35)+100(((1.004166667)^(36)-1)/.004166667)}}}
{{{FV = 4000(1.156652858)+100(.161472245/.004166667)}}}
{{{FV = 4626.611432+100(38.7533357)}}}
{{{FV = 4626.611432+3875.33357}}}
{{{FV = 8501.95}}}

<b>Her balance in 3 years is $8501.95.</b>

2.
{{{A =P(((r/n)(1+r/n)^(nt))/((1+r/n)^(nt)-1))}}}
{{{21=P(((.15/12)(1+.15/12)^((12)(3)))/((1+.15/12)^((12)(3))-1))}}}
{{{21=P(((.0125)(1.0125)^36)/((1.0125)^36-1))}}}
{{{21=P((.0125)(1.563943819)/.563943819)}}}
{{{21=P(.019549298/.563943819)}}}
{{{21=P(.034665329)}}}
{{{605.79 = P}}}

<b>The price of a set of encyclopedias is $605.79.</b>
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