Answer:
2 1/4 gallons
Explanation:
We assume your problem is intended to say there are five tanks, each having a volume of 1/2 gallon, and that 4.5 of these have been filled so far.
The volume of water used is ...
(4.5 tanks)(0.5 gal/tank) = 2.25 gal
2.25 gallons of water have been used.