what interest rate is required for an investment subject to continuous compounding to triple in 15 years?

Question

A= Pe^rt ?

Answer 1

e^15r = 3
15r = ln3
r = ln3/15 = 7.3%

Answer 2

I have the same question except mine is 5 years. Using this formula, I don't even come close to the correct answer in the book. Please advise.