How long will it take $10K to grow to...

Hello:

This is a basic finace question (or algebra?); I can't figure out how to solve this manually.

How long will it take for $10,000 to grow to $20,000 at an interest rate of 15% per year?

So:

20,000 = 10,000(1.15)^n

n=4.96 years

I can enter this in to my BA II Plus without issue (and this is fine for my class, but I just want to know what steps to take to work this out manually. I think I missed a whole section of basic math in my studies.

Any help is truly appreciated.