Hello.
The constant rate of change is 6 gallons. To figure this out, divide the amount of gallons it was filled to, and the minutes. 12 ÷ 2 = 6. It's constant rate would be: 6gal / 1min, 12gal / 2min, 18gal / 3min, 24gal / 4min, 30gal / 5min, etc.
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