After graduation you bought an electric car (thanks again for going green!!!) You will need 5.4 kWh for 18 miles of daily commute to work (roundtrip). Your old car needs a gallon of gasoline for this commute (14D).
How much days can you commute with the electric car with one gallon of gasoline?

The number of days you can commute with the electric car with one gallon of gasoline is 6.24 days

Explanation:

Electric car uses 5.4 KWh for 18 miles of daily commute to work.

Old car uses a gallon of gasoline for the same commute.

We need to convert a gallon of gasoline to KWh

The ratings for conversion will be based on EPA's (Gasoline Gallon Equivalent) formula, in which 33.7 kilowatt hours of electricity is equivalent to one gallon of gasoline.

1 gallon of gasoline = 33.7 KWh

5.4 KWh does the commute in 1 day,

33.7 KWh will do the commute in 33.7×1/5.4 = 6.24 days.