If you know that each large can is twice the size of a small can then you can form the equation x=2x meaning every one large can equals two small cans. That means that the 2 large cans are equal to four small cans, so altogether you have nine small cans. Between those nine you have 72 liters, so your equation should be 72=9x. To solve you do 72/9 which is 8. That means eight liters are in a small can and you can double it to find that sixteen liters are in a big can.
