It can be said that the total amount of fuel that was used by Michael when he was away to his friends was 12 gallons.
The question said that the tank was full before he left to his friends. This was after he checked the gauge of the gas tank.
The question says that the required quantity needed to fill up this tank is 24 gallon. Hence if it is full, then the total gallon it had before he made this trip was 24 gallon.
Then we are told that he went ahead to check the gauge when he finished and was at home. The quantity of fuel he had at this time was 1/2 of what was there.
This would be 1/2 * 24
12
Hence he made use of 12 gallons when he was away.
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