Answer:
he could drive 4 hours
Explanation:
if he drives for 1 hour he will have 21 gallons left
2 hours = 15 gallons left
3 hours = 9 gallons left
4 hours = 3 gallons left
4.5 hours = 0 gallons left
he could drive a total of 4.5 hours before he runs out of gas
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