Answer
c
Work Step by Step
a. 15 + 5 = 20, which is not 28 gallons.
b. 20 gallons is not 3 times as much as 8 gallons (8x3=24)
c. 21 gallons is 3 times as much as 7 gallons (7 x 3 = 21) and 21+7=28 total gallons
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