First we need to know the equivalence between gallons and cups, and that is:
1 gallon is 16 cups
Then we simplify the gallons fraction:
1 1/2 gallons, that is 3/2 gallons, do the math to verify, 1 whole is 2/2 plus the 1/2 extra that is 3/2
Now we have to multiply the amount of gallons we have times the cups them represent:
(3/2)gallons (16)cups per gallon
= (3/2)(16) = 3*16/2
= 24 cups
therefore, 16 cups can be served wit 1 1/2 gallons of soup