Final answer:
The correct answer is (d) 250.6 payments.
Step-by-step explanation:
To calculate the time it takes to repay the loan if Mr. & Mrs. Gruber double each payment, we first need to calculate the regular bi-weekly payment.
We can use the present value of an annuity formula to calculate the regular payment amount: PV = R * (1 - (1 + i)^-n) / i, where PV is the loan amount, R is the regular payment, i is the interest rate per period, and n is the total number of periods.
In this case, the loan amount is $500,000, the interest rate per period is 4% divided by 2 (due to semi-annual compounding), and the total number of periods is 25 years multiplied by 52 (due to bi-weekly payments).
Once we find the regular payment amount, we can double it and use the same formula to calculate the number of payments needed to fully repay the loan.