John made 2 quarts of juice for a school party. he said that he made 2/4 cups of juice. explain john's mistake.

Given: 
2 quarts of juice for a school party 
equivalent to 2/4 cups of juice.

This is wrong. Quartz refer to the part of a gallon. It means it is 1/4 of a gallon. 
1 gallon is equivalent to 16 cups.
1/4 of a gallon is equivalent to 4 cups.

2 quarts means 2 (1/4) = 1/2 gallon
1/2 gallon is equal to 8 cups.

John must be corrected. Instead of 2/4 cups of juice, it should be 8 cups of juice.