1 gallon = 16 cups. She is making 2 gallons so needs 32 total cups.
The ratio is 4:3:1
Add the ratios: 4+ 3 + 1 = 8
Divide total cups by total ratio:
32/8 =4
Now multiply that by each portion of the ratio:
Grape juice = 4 x 4 = 16 cups
Pineapple juice = 3x4 = 12 cups
Cider = 1x 4 = 4 cups
She needs 12 cups of pineapple juice.