
Walter B. answered 07/03/17
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Hi Arron,
This problem is most easily worked if thought of as two annuities.
First, we need to find the monthly payment that will payback the $160,000 at 3% interest with monthly payments for 25 years, similar to a $160,000 mortgage at 3%.
Terms:
Monthly interest rate (i) = annual rate =.03/12 = .0025
Number of compounding periods (n) = Term * 12 = 300 compounding periods
PMT = monthly payment
PV = present value
First, we need to find the monthly payment using the ordinary annuity formula for the present value.
PV = PMT * (1/i)*(1-1/(1+r)^n) or PMT*(1/.0025)*(1-1/(1.0025)^300)
Solve for PMT gives us
PMT = $160,000/(1/.0025)*(1-1/(1.0025)^300) = $758.74 per month
In order to find the balance after 7 years, we simply need to find the present value of 25 -7 or 18 years of cash flows.
now n = 18*12 or 216 payments so that the balance is just the present value of 216 payments discounted at .25%
PV= PMT *(1/.0025)*(1-1/(1.0025)^216) = $758.74 * 166.7435658 = $126,515.01
Hope this helps,