Could someone please help me with this problem:

You boil an egg. When you take the egg out of the boiling water, the temperature inside the egg is 100 Celsius. The temperature in the room is 20 Celsius. 120 seconds after the egg is taken out of the water, the temperature in the egg is 80 Celsius. We call the temperature difference between the temperature in the egg and the temperature in the room for D (t), where t is the time in seconds from the egg is taken out of the boiling water. We assume that D (t) decreases exponentially on the form: D (t) = ca^t

Determine c and a from the information given above. How long it takes from the egg is taken out of the boiling water to the temperature in it is 40 degrees Celsius?