Answer:
17 7/12 gallons of gas.
Explanation:
Set the fractions to like denominators. It'll make it a hell of a lot easier to add.
20 2/3rds = 20 8/12
9 1/4ths = 9 3/12
2 1/2 = 2 6/12
Adding those together gets you 31 17/12 which is 32 5/12 simplified.
Take that fraction and subtract it from your 50 gallons and it'll get you 17 7/12