How do I solve decibels for watts?

Question

The problem I'm trying to solve is this:

The input to an audio amplifier is 2 watts. It's output is 40W. What is the amplifier gain in dB?

I'm not asking for the answer, but you are welcome to give it to me as long as you can tell me how you got it.

Thanks for your help!!

Answer 1

Take the logarithm to base 10 of the input power and the logarithm of the output power.

Subtract log(Pout) from log(Pin).

Multiply by 10.

That is the answer.

Answer 2

Power gain in Bels is just log(power out/power in)

[log to base 10]

There are 10 decibels to a Bel, so

Power Gain = 10log(Pout/Pin)  db

10log(20) is about 13db

Edit:  Looks good.