1.#7 Page 618: A peach pie is removed from the oven at 5:00 PM. At that time it is piping hot, 100 C. At 5:10 PM its temperature is 80 C; at 5:20 PM it is 65 C. What is the temperature of the room?

The answer is 20 degrees Celsius but I'm not sure how to get there from this step

(0, 100): 100= x-u

(10, 80): 80 = x-ue^-10k

(20, 65): 65= x- ue^-20k

solving for x "simultaneously" is what yields 20 degrees Celsius but I don't know how to do that with those equations to get 20.

btw the original equation that got me to those 3 equations was dT/dt= -k(T-x) then you make it to dt/dT= 1/(k(x-T)) then separate variables etc

any help is apprecfiated