Answer:
434,470 gallons
Step-by-step explanation:
The exponential equation for the volume v remaining after t hours can be written as ...
v(t) = (initial amount)×(decay factor)^(t/(decay time))
v(t) = 500,000×(1 -10%)^(t/3)
Then after 4 hours, the remaining volume is ...
v(4) = 500,000×(0.90^(4/3)) ≈ 434,470 . . . . gallons
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Comment on the form of the equation
The decay factor is related to the decay rate by ...
decay factor = 1 - (decay rate)
and the decay time is the time applicable to that decay rate. Here, the rate is 10% every 3 hours, so the decay time is 3 hours for a decay rate of 10%.