Evin's car has fuel efficiency of 30 miles per gallon of gas used. if evin is driving at a constant 54 mph and her car starts with 14 gallons of gasoline, how many gallons are left in the tank after 3 hours of driving?

If the car traveled at a speed of 54 mph, then in three hours it traveled:

54 = 162 miles.

We know that for every 30 miles 1 gallon of fuel is spent, and we want to know how many gallons are spent in 162 miles.

If we call "Ge" the number of gallons spent in 162 miles, then:

Ge = 162

Ge = 5.4 gallons

If initially there were 14 gallons in the tank and 5.4 were spent, then  

14-5.4 = 8.6 gallons

After driving for 3 hours, 8.6 gallons of fuel remain in the tank