Keishas mom buhys 2 gallons of lemonade she serves 1 cup each gor 20 quests how many cups of lemonade remain

There are 16 cups in a gallon.  You have 2 gallons to begin with, so you have 32 cups to begin.

You give out 20 cups, so you only have 32-20 cups left.  32-20=12.  You have 12 cups left.