If the simple interest on a certain sum of money for 4 years is one–fifth of the sum, then the rate of interest per annum is
Choose the correct option.
A) 4 %	B) 7 %	C) 6 %	D) 5 %	E) 3 %

Rationale: "explanation : let the principal ( p ) be x then , simple interest ( si ) = x / 5 time ( t ) = 4 years rate of interest per annum ( r ) = ( 100 × si ) / pt = ( 100 × ( x / 5 ) / ( x × 4 ) = 20 / 4 = 5 % answer : option d"
Answer: D