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?

When you're trying to find an estimate, the best thing to do is to round them accordingly.
357.9 can be rounded to 360 (as it is over 5, therefore, it is rounded up)
21 can be rounded to 20 (as it is under 5, it is rounded down)

Then to find the miles per gallon, you need to divide the distance driven (360), by the amount of gas (20)
360/20= 18

As an estimate, Ellen gets 18 miles to the gallon.
(The actual answer is 357.9/21= 17.13 miles to the gallon)

Hope this helps :) 

21, i'm pretty sure it's right lol