Ever since an automobile started to have a mechanical problem, every time it has burned a gallon of gasoline, it has traveled only half as far as it did for the last gallon of gasoline it burned. if it traveled 40 miles for the first gallon of gasoline, it burned since the problem started, and if it has burned a total of 5 gallons of gasoline since the problem started, how many miles per gallon has the automobile averaged since the problem started?

a)
15 miles per gallon

b)
15.5 miles per gallon

c)
16 miles per gallon

d)
16.5 miles per gallon