A car traveled 420 miles per tankful of gasoline on the highway and 336 miles per tankful of gasoline in the city. If the car traveled 6 fewer miles per gallon in the city than on the highway, how many miles per gallon did the car travel in the city?	Pick one
A) 14	B) 16	C) 21	D) 24	E) 27

Rationale: "let the speed in highway be h mpg and in city be c mpg . h = c + 6 h miles are covered in 1 gallon 420 miles will be covered in 420 / h . similarly c miles are covered in 1 gallon 336 miles will be covered in 336 / c . both should be same ( as car ' s fuel capacity does not change with speed ) = > 336 / c = 420 / h = > 336 / c = 420 / ( c + 6 ) = > 336 c + 336 * 6 = 420 c = > c = 336 * 6 / 84 = 24 answer d ."
Answer: D