The Helvie family drinks 7/18 of a gallon of milk per day.
Divide the total amount of milk drunk by the number of days.
1 3/4 / 4 1/2
7/4 / 9/2
Division by a fraction is the same as multiplication after swapping the top and bottom of the fraction
7/4 * 2/9 = 14/36 = 7/18
