The vehical's fuel efficiency is more than 40 miles per gallon

use f to represent the vehicle's fuel efficiency (in miles per gallon)

"The fuel efficiency is more than 40 miles per gallon". If f is the fuel efficiency, then we can replace all of the phrase "the fuel efficiency" with the letter f to have this new sentence be "f is more than 40 miles per gallon". All f is doing is standing in place of a more complicated idea. It's simplifying things down for us.

Replace the "is more than" with a "greater than" sign and we end up with f > 40 as our final answer, assuming your teacher just wants you to give the inequality format