The formula for simple interest is I=PRT, where I = interest earned, P = principal amount invested/borrowed, R = rate as a decimal, and T = time in years.
In order to find the rate, we need to know the interest earned. So we’ll subtract the information given to find what the interest earned was: 4000-3200 = 800
I = PRT
800= (3200)(r)(4)
800= 12,800r
r = 0.0625
Multiply by 100 to get percentage
r = 6.25%
Please let me know if you have questions about this answer.