9.11 Consider Example 9.4. Suppose the tank is operating in the steady state of part (a) with all flow rates and amounts as indicated in the problem statement, T1=20C, and T=T2=30C. Now the heater is shut off. Assuming the tank is perfectly insulated from the surroundings, write and solve the differential equation for the tank temperature T(t). How many minutes will it take for the water in the tank to fall to 25C ? 9.12 Consider Problem 9.11. Suppose now that the tank is not insulated from the surroundings but transfers heat to the surrounding air (at 20C ) with an overall heat transfer coefficient U=10kJ/minm2C. The heat transfer area is 2m2. Write and solve the differential equation for the tank temperature T(t). How many minutes