A person 'X' invests 'P' dollars at once. In 7.25 years, if the amount doubles, then what would be the amount in terms of 'P' for 29 years?

So, I see that every 7.25 years we have P+P. Now, 7.25 goes into 29 4 times. Thus, we should have (P+P)^4=(2P)^4=16P^2.

how does this look?