Total quantity of water that Jan needed to empty = 18 gallons
There are other important information's already given in the question. Based on those information's the answer can be easily deduced.
Number of jugs that can hold 0.5 gallons of water = 1
Then
Number of jugs required to hold 18 gallons of water = 18/0.5
= 36
So 36 jugs are required by Jan to empty 18 gallons of water.