This is part of a larger mixing problem, but I'm confused with this particular part. I got to an integral of:

$\displaystyle \int{\frac{1}{30-A}dA}$ which to me is pretty straight-forward as just $\displaystyle ln|30-A|$. For some reason, the book says the answer is $\displaystyle -ln|30-A|$. Can someone explain where the negative came from?

I can post the full problem if that's important.