If there are 4 parts of pineapple juice to 1 part of orange juice how much orange juice is needed to make 20 quarts of punch?

Short answer: 4 quarts

Long (and more complete) answer:

# of parts total = 4 + 1 = 5

total beverage quantity = 20 quarts

beverage quantity per quart = 20 / 5 = 4

So, 4 parts pineApple and 1 part orange = 16 quarts pineApple, 4 quarts orange.

If there's enough of any other ingredients such that they add up to be as much as 1 "part", you'd have to include them in the calculation. But if it's just those two ingredients, 4 quarts OJ.