If there are 16 cups in one gallon, each gallon u add 16. If you have 3 gallons you add 16 3 times. So to get your answer you multiply 16 by three which is 48 cups.
Since 1 gallon= 16 cups, to find out how many cups are in 3 gallons, you would do 16+16+16, each 16 added representing 1 gallon, and you would get 48 cups. To find how many cups are in 5 gallons, you would use the same formula you did to find out how many cups are in 3 gallons. So, you would do 16+16+16+16+16, each 16 added representing 1 gallon, and you would get 80 cups.
