There are 20 cups and each one holds 10 fluid ounces. Assuming that she wants to fill them all up to the brim and use the complete 10 fluid ounces for each cup this means she would have a total of 200 fluid ounces: 20 * 10 = 200
1 gallon = 4 quarts
1 quart = 32 fluid ounces
This means that for every gallon she has 128 fluid ounces: 1 gallon (4 quarts/1gallon)(32 fluid ounces/1 quart) = 128
This means she needs at least 200 fluid ounces and she must buy another gallon to satisfy this; 128 * 2 = 256 fluid ounces.
She needs to buy 2 gallons but will have an extra 56 fluid ounces