Start with the most straightforward piece of information and translate it into math. In this case, the most straightforward information is about the total volume of the balloon. "Between the two gases" would indicate addition, and the gases are represented as x and y, so you need x+y in the correct answer. They all have that, so go to the next piece. Together, x and y can be "up to 8 liters." This translates to "less than or equal to 8," because the balloon's volume can't be greater than 8, but it could be 8 or less. Therefore, one equation must be x+y≤ 8. You can eliminate (A) and (B), as those answers have different equations related to x+y. The next part may be hard to translate, so pick some numbers and try them out. Make x=2 and y=2. The second equation in (D) is easier to work with, so see if these numbers make that equation true. 0.2(2)+1.30(2)= 0.4+2.60=3. This is not less than 1.20, so eliminate (D) and choose (C).