You need to find a value for k and A. You know that the initial temperature is 10 C, so

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.)

-Dan