Suppose there are two tanks, each containing 20L of fluid. Tank 1 initially contains water with 4kg of salt dissolved and Tank 2 initially contains pure water. The tanks are stirred constantly so their solutions have uniform concentration. There is a pipe which takes 15 L/min from tank 1 to tank 2, and another pipe which takes 15 L/min from tank 2 to tank 1. Also, pure water is being pumped into each of tank 1 and tank 2 at a rate of 5 L/min, and solution is being removed at a rate of 5 L/min from each of tank 1 and tank 2. Model the initial value problem as a system of two first order differential equations, solve the initial value problem, and find the amount of salt in tank 1 after one minute.