jmc

Call the number of one dollar bills $x$ and the number of two dollar bills $y$. We can use the following system of equations to represent the given information: $$\begin{array}{rl}x+y& =49,\\ 1x+2y& =66.\end{array}$$The first equation represents the total number of dollar bills in the piggy bank, and the second equation represents how much money is in the piggy bank. Solving for $x$ in the first equation gives $x=49-y$. Substituting for $x$ in the second equation yields $49-y+2y=66$, or $y=17$. But $y$ is the number of two dollar bills, and the question asks for the number of one dollar bills, so solve for $x$: $x=49-17$. Thus, there are $\boxed{32}$ one dollar bills.