The usual way to find the compound interest is given by the formula A = .p(1+r/100)^n
In this formula,
A is the amount at the end of the period of investment
P is the principal that is invested
r is the rate of interest in % p.a
And n is the number of years for which the principal has been invested.
In this case, it would turn out to be A =1500(1+20/100)^3