Ok so, here goes:
simple interest: r- rate, A- amount in the end, t- time, P- principal
r = (1/t)(A/P - 1)
Putting in our numbers
r = (1/4)((21236/5309) - 1) = 0.75
r = 0.75
Converting r decimal to R a percentage
R = 0.75 * 100 = 75%/year
The interest rate required to get a total amount, principal plus interest, of $21,236.00 from simple interest on a principal of $5,309.00 over 4 years is 75% per year.
