Maria buys computer disks at a price of $4$ for $$5$ and sells them at a price of $3$ for $$5$.  How many computer disks must she sell in order to make a profit of $$100$?
$\text{(A)}\ 100 \qquad \text{(B)}\ 120 \qquad \text{(C)}\ 200 \qquad \text{(D)}\ 240 \qquad \text{(E)}\ 1200$
This is the equivalent of saying she buys $12$ for $$15$ and sells $12$ for $$20$, so for every dozen disks she sells, she profits $$5$.
She needs to profit $$100$, so she needs to sell $\frac{100}{5}=20$ dozen disks, which is $240\rightarrow \boxed{\text{D}}$