This is a thermodynamics question. I am the most confused about part b.

Please explain your answer. Thank you

A mixing tank initially contains 50kg of water at 25C. Suddenly, two inlet valves and one outlet valve are opened. Once the valves open, there are two inlet streams of water each having a flow of 5 kg/min into the tank; and there is one outlet stream of water at a flow of 10kg/min leaving the tank. The temperature of one inlet stream is 80C, and the temperature of the other is 50C. The tank is well-mixed, so that the temperature of the outlet stream is always the same as the temperature of the water in the tank. Assume that the stirring in the tank does not significantly affect the energy of the water in the tank. a) Compute the steady-sate temperature that will finally be obtained in the tank. b) Develop an expression for the temperature of the fluid in the tank at any time. Note: this expression must be of the form T=f(t)