At what rate percent per annum will sum of money double in 20 years?  Options:  A) 1.2 %  B) 2 %  C) 4 %  D) 5 %  E) none of these

Rationale: "explanation : hint : if sum of money becomes ( z times ) in ( t ) years at simple interest , then rate of interest ( r ) can be calculated using the formula : rate of interest ( r ) % = 100 ( z – 1 ) / t here , principal amount is not given . hence , we can directly use the trick to calculate the rate of interest . rate of interest ( r ) % = 100 ( 2 – 1 ) / 20 rate of interest ( r ) % = 5 % p . a . answer is d"
Answer: D