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Core 4 !! Rates watch

1. Hey, please can someone explain this question to me because i really don't understand how to get the correct answer:

8. Liquid is pouring into a container at a constant rate of 20 cm^3 s–1 and is leaking out at a rate proportional to the volume of the liquid already in the container.

(a) Explain why, at time t seconds, the volume, V cm3, of liquid in the container satisfies the differential equation

dV/dt = 20 – kV,

where k is a positive constant.
2. The rate of change of liquid in the container (dv/dt) is equal to the rate of water going in minus the rate of water leaking out.

Where are you stuck?

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