By Newton's law of cooling you have that the temperature of the water is given by:
where is the temperature of the fridge and r is some unknown rate constant. You are told T(10) = 39 and T(20) = 33, so:
The first equation gives you:
and the second gives you:
Equating these two expressions for r and multiplying by -20 yields:
The 2 can move inside the log as a power, then you can get rid of the logs and solve. You should get .