Question

So I don't really understand how to solve this.. Here's the question.

Answer 1

The answer is 9 gallons.

  ? - 8 + 10 -5 = 6
Then fill in the choices.

3-8+10-5=0
7-8+10-5=4
9-8+10-5=6
11-8+10-5=8

Answer 2

First lets see what information they give us.
We know that they arrived with 6 gallons of gas and along the way they pumped 18 gallons and used 5.
(i got 18 by adding 8 and 10 just to make it simpler)