If at the end of each year you will receive only $50, then the investment is in simple interest only. It is not compounding.

A = P + r*P*t

A = P +(r*P)t

A = P(1 +rt)

where

A = final amount after t years.

P = principal or initial amount.

r = interest rate per annum, or yearly rate of return.

t = time in years

The "r*P" in your question is $50.

So,

r*P = 50

r(320) = 50

r = 50/320 = 0.15625

r = 15.625 percent per annum ---------------answer.