In Exercises consider a tank that at time t = 0 contains v₀ gallons of a solution of which, by weight, q₀ pounds is soluble concentrate. Another solution containing q₁ pounds of the concentrate per gallon is running into the tank at the rate of r₁, gallons per minute. The solution in the tank is kept well stirred and is withdrawn at the rate of r₂ gallons per minute.

Let Q be the amount of concentrate in the solution at any time t. Write the differential equation for the rate of change of Q with respect to t when r₁ = r₂ = r.