I need to code to get the solve it, but I just need help with setting up the pro

Question

I need to code to get the solve it, but I just need help with setting up the problem. Can someone transform this problem statement into a ODE? (Differential equation) Thanks!

Problem 1: Mixing During the start-up of a process, a tank with initially 100 liters of a NaCl solution that contains 4 kg of salt, is fed with pure water at a rate of 5 L/min and discharged at the same volumetric rate. Assuming perfect mixing and a constant temperature, calculate how much NaCl (in kg) is still remaining inside the tank after 30 min of operation. Use Runge-Kutta 2nd order: Heun Method and step size = 1 min.

Explanation / Answer

Q'(t) = inflow - outflow = 0* r - (Q(t)/V))*r

Since no inflow is there of NaCl concentration , because the water is pure

For outflow , the concentration at any time t , is Q(t)/ V

Hence the amount of NaCl going out is concentration * rate = Q(t)/ V * r

V = 100 ;

r = 5L/min

Hence , Q'(t) = - (Q(t)/V))*r = -Q/100 * 5 = -Q/20

Differential Equation is : Q'(t) = -Q/20

with Q(0) = 4 , you have to find Q(30)

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