1/2 is equal to 5/10.
When Mai removes 200 pennies from her penny bank, the container drops from 7/10 full to 5/10 full.
7/10-5/10=2/10=1/5
200 pennies is 1/5 the capacity of the penny bank.
1/5 of the bank is 200 pennies, so a full bank is 1000 pennies.