So if the bank was 2/3 full, and then after the pennies are removed it is 1/2 full, then the volume taken up by those removed pennies is the difference:
(2/3)-(1/2)=(4/6)-(3/6)=1/6
If there are 120 pennies in 1/6 of the bank's volume, then the total can be found by multiplying 120 by 6 (so that (1/6)*6=1 (the total volume))
120*6=720
You can fit 720 pennies in that bank