1. Milk costs x cents per half-gallon and y cents per gallon
Milk costs x cents per half-gallon and y cents per gallon. If a gallon of milk costs z cents less than 2 half-gallons, which of the following equations must be true?
Difficulty:
Medium
x – 2y + z = 0
2x – y + z = 0
x – y – z = 0
2x – y – z = 0
x + 2y – z = 0
HINT
If your answer does not appear as one of the choices, try to determine if any of the choices is equivalent to your answer.

2. D: 2x – y – z = 0
Explanation
The cost of a gallon of milk (y cents) is equal to z cents less than the cost of 2 half-gallons (2x cents). This can be written algebraically as y = 2x – z. Subtracting y from both sides of the equation yields 2x – y – z = 0.