A lake contains 40 kg of polluting matter dissolved in 250 kL (thousands of liters) of water. Polluting matter enters the lake dissolved in water from melted snow, as a solution with a concentration of 0.2 kg/kL at a rate of 10 kL/day. At the same time, the solution of water from the lake flows out into a river also at a rate of 10 kL/day, so that the volume of the lake remains constant. Assume the solution of water in the lake is well-mixed at all times. Find an algebraic expression for the total amount of polluting matter in the lake (in kilograms) as a function of time (measured in days).
Provide your answer below:
u(t) =