How Long Will It Take An Investment To Quadruple If It Earns 8 Compounded Semi A

 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 =P

Then the future value of the investment which is A= 4P

The rate of the interest; r=8%=0.08

The number of compounding period per year= 2

So let time token= t years

In order to solve this problem I will be using the compound interest formula;

A= P(1+r)^nt

4P= P (1+0.08/2)^2t

4= (1.04)^2t

Ln4= 2t In(1.04)

t=In (4)/ 2 In (1.04)

= 17.67 which is approximately equal to = 18 years 

final answer, 18 years