At the age of 34

​,
to save for​ retirement, you decide to deposit ​$80

at the end of each month in an IRA that pays 6
​%
compounded monthly.
a.
Use the following formula to determine how much you will have in the IRA when you retire at age 65.
Upper A equals StartFraction Upper P left bracket left parenthesis 1 plus r right parenthesis Superscript t Baseline minus 1 right bracket Over r EndFraction
    
or
Upper A equals StartStartFraction Upper P left bracket left parenthesis 1 plus StartFraction r Over n EndFraction right parenthesis Superscript nt Baseline minus 1 right bracket OverOver left parenthesis StartFraction r Over n EndFraction right parenthesis EndEndFraction
b.
Find the interest.

1 answer

a. In this problem, P = $80, n = 12 (since there are 12 months in a year), r = 6%/100 = 0.06 (as r should be in decimal form), and t = 65 - 34 = 31 years.

Let's use the second formula:
A = P * [ ((1 + (r/n))^(nt) - 1 ) / (r/n) ]

Substituting the given values, we get:
A = 80 * [ ((1 + (0.06/12))^(12*31) - 1 ) / (0.06/12) ]

Calculate the expression inside the brackets first.
A = 80 * 97.9233

Multiply the two:
A = $7,833.86

So, when you retire at age 65, you will have approximately $7,833.86 in your IRA.

b. The interest earned on the account is the final account balance minus the total contributions. You made monthly contributions over 31 years, which is 31*12 = 372 months.

Compute the total of your contributions by multiplying the number of months by the amount of contribution per month.
So, total contributions = $80 * 372 = $29,760

Subtract this from the final account balance to find the interest.
Interest = $78,338.66 - $29,760 = $48,578.66