If the simple interest on a certain sum of money for 6 years is one–fifth of the sum, then the rate of interest per annum is  Pick one: A) 3.3 % B) 7.2 % C) 6.1 % D) 5.0 % E) 3.1 %

Rationale: "explanation : let the principal ( p ) be x then , simple interest ( si ) = x / 5 time ( t ) = 6 years rate of interest per annum ( r ) = ( 100 × si ) / pt = ( 100 × ( x / 5 ) / ( x × 6 ) = 20 / 6 = 3.3 % answer : option a"
Answer: A