In how many years will a sum of money doubles itself at 18% per annum on simple interest?  Select the correct option
A) 5.9 % B) 5.5 % C) 6 % D) 6.4 % E) 7.5 %

Rationale: "p = ( p * 18 * r ) / 100 r = 5.5 % answer : b"
Answer: B