Diethyl carbonate, $(\mathrm{C}{}_2\mathrm{H}{}_5\mathrm{O}){}_2\mathrm{C}=\mathrm{O}$, is treated with excess $\mathrm{C}{}_2\mathrm{H}{}_5\mathrm{MgBr}$, then $\mathrm{H}{}_3\mathrm{O}{}^{+}$. The final product is

Three C-O/C=O links are replaced in succession: the first $\mathrm{C}{}_2\mathrm{H}{}_5\mathrm{MgBr}$ expels ethoxide to give ethyl propanoate, the second gives pentan-3-one, the third gives $(\mathrm{C}{}_2\mathrm{H}{}_5){}_3\mathrm{C}-\mathrm{OMgBr}$. Hydrolysis gives 3-ethylpentan-3-ol, which uses 3 mol of Grignard.