N = 2424 is the sample size (amount of cars being sampled)
df = degrees of freedom
df = n-1
df = 2424-1
df = 2423
Side Note: if there is a typo and the sample size should be n = 24 (instead of 2424), then the df would be df = n-1=24-1 = 23
The anser is b guess and test
He uses 15 gallons. Because 270/6 = 45. Meaning he drives 45 miles per gallon. So 675/45=15. 15 gallons he will need to use to drive 675 miles.