To find out how many jugs Jan needs to empty 18 gallons of water into jugs that each hold 0.5 gallons, you can divide the total volume of water by the capacity of each jug.
\[ \text{Number of jugs} = \frac{\text{Total gallons}}{\text{Capacity of each jug}} = \frac{18 \text{ gal}}{0.5 \text{ gal/jug}} \]
Calculating that gives:
\[ \text{Number of jugs} = 18 \div 0.5 = 36 \]
Therefore, Jan needs 36 jugs to empty 18 gallons of water from the tub.