Q: Petrol costs x dollars half a gallon and y dollars per gallon. If a gallon of petrol costs 2 dollars less than 2 half-gallons, which of the following equations must be true?
a. x - 2y + z = 0
b. 2x - y + z = 0
c. x - y - z = 0
d. x + 2y - z = 0
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