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 .