Ellen drove 357.9 miles. her car gets about 21 miles per gallon. which is the best estimate of how many gallons of gas ellen used?

The best thing to do is to divide the amount she has driven (357.9) by the miles (21)
As you're looking for an estimate, you have to round them.
357.9 rounds to 360
21 rounds to 20
Therefore, you've got to divide them
360/20= 18
Therefore, Ellen has driven approximately 18 miles
Hope this helps :)

Ithink the answer is c