Question

(Really Hard Question)
Mixing. Suppose a brine containing 0.3 kilogram
(kg) of salt per liter (L) runs into a tank initially
filled with 400 L of water containing 2 kg of salt. If
the brine enters at 10 L/min, the mixture is kept uniform by stirring, and the mixture flows out at the same rate. Find the mass of salt in the tank after 10 min [Hint: Let A denote the number
of kilograms of salt in the tank at t min after the process begins and use the fact that rate of increase in A  rate of input  rate of exit.

Answer 1

First, you need to figure out how much salt comes in per minute.
Can you do that ?

Answer 2

dA/dt=3 - A(t)/40

is that right?

Answer 3

Yes, that looks good.
The amount of salt in the tank will be
A = 2 (starting amount) + 3 t (amount coming in) - A*t/40 (amount leaving)

replace t with 10, and solve for A.