If a sum of money doubles itself in 5 years at simple interest, the ratepercent per annum is  Choose one:
A) 12
B) 12.5
C) 20
D) 13.5
E) 14

Rationale: "explanation : let sum = x then simple interest = x rate = ( 100 * x ) / ( x * 5 ) = 20 option c"
Answer: C