a.) if 4oz is drank every 4 hours, and there are 24 hours in a day, then there are 6 times during each day that the baby drinks formula. Knowing this, multiply 6 and 4 to get 24oz of formula each day
b.) if 1 can = 91fl oz, and, let's say 30 days in a month, then 24 multiplied by 30 will gives us how much is drank each month which is equal to 720fl oz. Divide 720 by 91 to get approximately 8 cans for one month.
c.) Go back to sub-problem a to find that the new amount per day is now equal to 16oz (6oz/4hrs ... 24/6 = 4 ... 4*4 = 16). Now take 30 and multiply it by 16 to get 480fl oz. Divide 480 by 91 to get approximately 5 cans for one month.