If a sum of money doubles itself in 20 years at simple interest, the ratepercent per annum is
Choose the correct choice from the following choices.
A) 12  B) 12.5  C) 13  D) 5  E) 14

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