7. Sally's Summer Punch recipe is a combination of lemonade and fruit punch: Sally uses 3 cups
of lemonade for every 4 cups of fruit punch. In order to make a batch for the Fourth of July
party, Sally uses 21 cups of lemonade. How many cups of fruit punch will Sally use?

LEMONADE |————————————|—>
3 CUPS

FRUIT PUNCH |———————————|—->
4 CUPS