"given an interest rate of 7.15 percent per year, what is the value at year t = 8 of a perpetual stream of $3,513 payments that begin at year t = 20"

This is a two part question.

First solve for the Present Value of a perpetual stream of $3513 payments, using a discount rate of 7.15%.

This calculation is pretty simple as the formula is:

Payment Amount / Interest rate
3,513÷0.0715=49,132.867132867

Then, since you won't start receiving the annual payments for another 12 years (T=20 vs T=8), you need to discount that figure back another 12 years.

That formula is

PV=FV / (1 + i)^n
PV=49,132.867132867÷(1+0.0715)^(12)
PV=21,451.91 round your answer to get 21452

What are the options?