The difference between the fraction originally contained in the bank (1/2) and the fraction that remains after Maria removes 120 pennies (1/3) represents the fraction occupied by the 120 pennies.
Therefore the fraction occupied by the 120 pennies= 1/2-1/3=1/6
thus if 1/6 of the bank can hold 120 pennies, then, when full it holds 120×6/1=720 pennies