STEP 1:
convert gallons to cups & calculate total number of cups
1 gallon= 16 cups
= 8 gallons * 16 cups/gallon
= 128 cups total
STEP 2:
calculate total cups estimated for 20 guests
= 20 guests * 2 cups/guest
= 40 cups needed for guests
STEP 3:
subtract the difference between step 1 and step 2 to find leftover punch
= 128 cups - 40 cups
= 88 cups leftover
OR
GALLONS LEFTOVER
= 88 ÷ 16 cups/gallon= 5 1/2 gallons
ANSWER: There will be 88 cups or 5 1/2 gallons of punch leftover.
Hope this helps! :)