2 gallons equals = 16/8 gallons
she used 3/8 gallon
The amount of milk that is left: 16/8 - 3/8 = 13/8 gallons of milk.
Basically, because the amount of used milk is in eights, you need to multiply and divide 2 gallons with 8, so you get 16/8 gallons. You then subtract what she used and you will get how much milk is left.