One tiny little gripe first. The line in red is written incorrectly. It should be:

But the rest is correct.

So, the problem is to solve

I'm going to assume you know logarithms, otherwise you can't solve this. The tricky part is how to do a log with a base of 2.5. I'm not even going to try! 2.5 = 5/2 so: (Edit: Come to think of it you can leave it as 2.5. I was going to do something with the fraction that I realized later I didn't need to do.)

(Or use log base 10, or your favorite base.)

(with a BIG sigh of relief that it's an integer...)

-Dan