Question

A contains 200 liters of solution in which is dissolved 40 kg of
salt. Tank B contains 400 liters of solution in which are dissolved
80 kg of salt. Pure water flows into tank A at rate of 10 liters per
second. There is a drain at the bottom of tank A. Solution leaves
tank A via this drain at a rate of 10 liters per second and flows
immediately into tank B at the same rate. A drain at the bottom of
tank B allows the solution to leave tank B, also at a rate of 10
liters per second. What is the salt content in the tank B after 1
minute?
x(t) :amount of salt in tank A
dx/dt: rate of salt changing with respect to time in tank A
y(t) : amount of salt in tank B
dy/dt: rate of salt changing with respect to time in tank B

rate of change of substance = rate in of substance

rate in/out =flow rate(or volume rate) in/out x concentration within the fluid entering/exiting

concentration = amount of salt/volume of solution

Answer 1

see solution 3 http://www.math.cmu.edu/~doffner/teaching/122/quiz7sol.pdf