Principal amount = P
Simple interest case:
Sum after 15 years = P (1 + 15 r ) = 3 P
=> 1 + 15 r = 3 => r = 2/15 or 200/15 % = 13.33 %
Compound interest case:
sum after 15 years = P ( 1 + r )^15 = 3 P
=> (1 + r )^15 = 3 1 + r = 3^1/15 = 1.076
=> r = 0.076 = 7.6 %