Jan needed to empty 18 gallons of water from a tub into jugs that hold 0.5 gallons of water. How many jugs did Jan need?

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.