Answer:
D. 1200 gallons
Explanation:
Fractions of the total are given, as is the quantity the difference of those fractions represents. These can be used to write an equation to find the total.
Setup
Let g represent the total number of gallons of fuel sold. Then the number of gallons of 87-octane sold is 0.50g, and the number of gallons of 89-octane sold is 0.35g. The difference is said to be 180 gallons:
0.50g -0.35g = 180
Solution
Collecting terms gives ...
0.15g = 180
Dividing by the coefficient of g, we find ...
g = 180/0.15 = 1200
The gas station sold 1200 gallons of fuel last weekend.
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Additional comment
That's 600 gallons of 87-octane; 420 gallons of 89-octane; 120 gallons of 92-octane; and 60 gallons of 99-octane.