First lets change the fractions so they have common denominators:
3/5 = 6/10
The bank was 1/10, after adding the pennies it was 6/10
6/10 - 1/10 = 5/10 = 1/2
This means 400 pennies filled 1/2 the piggy bank.
There are 2 halves (1/2) to a whole ( full piggy bank).
The piggy bank can hold 400 x 2 = 800 pennies.