and you know that the temperature is 15 C at t = 20 min. Thus
The first equation gives a value for A:
Thus A = 20. (The unit is degrees C.)
So the second equation gives a value for k:
If you want decimals, k is about 0.02554128.
So at t = 40 min...
(It's a good exercise to carry this through with the exact value of k.)