jmc

Peter ate $1+\frac{1}{2}=\frac{3}{2}$ slices. The pizza has $12$ slices total. Taking the ratio of the amount of slices Peter ate to the amount of slices in the pizza, we find that Peter ate $\frac{\frac{3}{2}\text{ slices}}{12\text{ slices}}=\boxed{\frac{1}{8}}$ of the pizza.