Replying to Intelligent economics NSers? (macro problem while studying)

hey guys, continuing studying for my macro economics final at UBC vancouver tomorrow, cant figure out this question. i thought i needed to use the growth formula but i just don't know what im doing. if anyone has time please give it a try and explain how you got the asnwer? much appreciated.




2. If real GDP in Canada grows at 4.0% per year, how many years will it take for it to triple?

A. about 12 years
B. about 20.5 years
C. about 28 years
D. about 38.5 years
E. it is impossible to tell without knowing population growth rate






like I said, I have the answer but just don't know how to get there!

thanks