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