Rounded to the nearest quarter year, how long will it take an investment to quadruple if it earns 8%

Rounded to the nearest quarter year, how long will it take an investment to quadruple if it earns 8% compounded semi-annually? did I did the calculations right or am I missing something ?So assuming that the initial investment is =PThen the future value of the investment which is A= 4PThe rate of the interest; r=8%=0.08The number of compounding period per year= 2So let time token= t yearsIn order to solve this problem I will be using the compound interest formula;A= P(1+r)^nt4P= P (1+0.08/2)^2t4= (1.04)^2tLn4= 2t In(1.04)t=In (4)/ 2 In (1.04)= 17.67 which is approximately equal to = 18 years final answer, 18 years