At what rate percent per annum will a sum of money double in 6 years.  Choose the correct choice:
A) 12.5 %  B) 13.5 %  C) 11.5 %  D) 14.5 %  E) 16.6 %

Rationale: "let principal = p , then , s . i . = p and time = 8 years rate = [ ( 100 x p ) / ( p x 6 ) ] % = 16.6 % per annum . answer : e"
Answer: E