jmc

This is the equivalent of saying she buys $12$ for $\text{textdollar}15$ and sells $12$ for $\text{textdollar}20$, so for every dozen disks she sells, she profits $\text{textdollar}5$. She needs to profit $\text{textdollar}100$, so she needs to sell $\frac{100}{5}=20$ dozen disks, which is $240\to \boxed{240}$