Question

Petra invest $10800after4years she has earned $3240 in interest ar what annual rate of interest did she invest her money?

Answer 1

let the annual rate be i

10800(1+i)^4 - 10800 = 3240
10800(1+i)^4 = 14040
(1+i)^4 = 1.3
take 4th root of both sides
1+i =1.06779
i = .06779 or appr 6.78% per annum compounded annually.

I assumed you wanted compound interest, since this is 2022.