so
8 ounce=1 cup
2 cups=1pint
2 pints=1 quart
4 quarts=1 gallon
128 ounces in 1 gallon
so she needs to get 10 times 20 ounces (200 oz) so
she will need to purchase (200/128) 1.5625 gallons
normally you cannot purchase a fraction of a gallon so she sill have to purchase 2 gallons