Brianna is using part of the money she earned on her weekend job to buy several equally-priced CDs. She used one fifth of her money to buy one third of the CDs. What fraction of her money will she have left after she buys all the CDs? 
$\textbf{(A) }\ \frac15 \qquad\textbf{(B) }\ \frac13 \qquad\textbf{(C) }\ \frac25 \qquad\textbf{(D) }\ \frac23 \qquad\textbf{(E) }\ \frac45$
Let $m =$ Brianna's money.  We have $\frac15 m = \frac13 (\mbox{CDs}) \Rightarrow \frac35 m = (\mbox{CDs})$.  Thus, the money left over is $m-\frac35m = \frac25m$, so the answer is $\boxed{\textbf{(C) }\frac{2}{5}}$.
This was just a simple manipulation of the equation. No solving was needed!