#### At what rate percent per annum simple interest will a sum triple itself in 15 years?

Let the sum be Rs x. Then,

P= Rs x, A=Rs 3x, SI=A-P= $(3x-x)=2x$  and T=15 years.

$\therefore R=\frac{100\times S.I.}{P\times T}=(\frac{100\times 2x}{x\times 15})%$

$\therefore R=\frac{40}{3}%=13\tfrac{1}{3}%$

Hence the required rate is $13\tfrac{1}{3}%$ p.a.