A sum of money, put at simple interest trebles itself in 15 years. The rate per cent per annum is :

A sum of money, put at simple 

interest trebles itself in 15 years.

 The rate per cent per annum is : 

(a) 40/3 %
(b) 50/3 %
(c) 38/3 %
(d) 20 %

Correct answer: a

Solution :