In how many years will a sum of money double itself at 18% simple interest per annum?

Correct option is C

Given:
Amount = 2 × Principal
Rate = 18% per annum
Formula Used:
(i) SI = PRT/100
(ii) A = SI + P
Where A = Amount
SI = simple interest
P = principal
R = rate
T = time
Solution:
SI = (P × R × T)/100
=> SI = (P × 18 × T)/100
=> SI = 9PT/50
A = SI + P
=> 2P = (9PT/50) + P
=> 2 = (9T/50) + 1
=> 1 = 9T/50
=> T = 50/9 years
=> T = 5 years and 6.67 months
∴ In 5 years and 6.67 months, the amount will be doubled at a rate of 18% simple interest per year.