Miriam,

First of all I would work in cents rather than dollars. Hence the piggy bank contains 625 cents.

Suppose the number of nickels is n, the number of quarters is q and the number of dimes is d. Since a nickel is worth 5 cents, a quarter is worth 25 cents and a dime is worth 10 cents

5n + 25q + 10d = 625.

The problem contains enough information to construct two more equations. Solve the three equations for n, q and d.

If you need more help write back,
Penny