40 gallons / 5 minutes = X gallons / 20 minutes:
Cross multiply:
40 * 20 = 5 *x
800 = 5x
Divide both sides by 5:
x = 800/5
x = 160 gallons after 20 minutes.
There are 3 20 minute periods in an hour
160 x 3 = 480 gallons in one hour.
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