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Sacha drains the water from a hot tub. The hot tub holds $1600$ L of water. It takes $2$ h for the water to drain completely. The volume of water in the hot tub is modelled by $V(t) = 1600 - \dfrac{t^2}{9}$, where $V$ is the volume in litres at $t$ mins and $0 < t < 120$.

a) Determine the avg rate of change in volume during the second hour.

I know the answer is $20$ L/min after looking at it from the answers section of my text book, but I dont know how they got that answer.

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    One way I guide myself through these kind of problems is to observe the units. If you know the rate is measured litres/min, how would compute L litres and divide it out by 60 minutes (= second hour)? What should be those L litres? See answers below.2012-08-04

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