Write a system of equations for each problem, and then solve the system.
Jonathan, a second grader, counted the money in his piggy bank. He had only quarters and dimes. When he added up his money, he had 39 coins worth a total of $\$ 7.50 .$ How many coins of each kind did he have?
$$\begin{array}{|c|c|c|}\hline \text { Number } & {\text { Value }} & {\text { Total }} \\ {\text { of Coins }} & {\text { per Coin }} & {\text { Value }} \\ \hline x & {50.25} & {} \\ {y} & {50.10} & {} \\ {\text { 39 }} & {\times \times \times \times} & {57.50} \\ \hline\end{array}$$