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.