On a hot summer day, a cool drink is removed from a refrigerator whose temperature is 10 degrees Celsius and is placed in a room where the temperature is 30 degrees Celsius. The temperature of the drink after t minutes is given by a function of the form f(t) = 30 - Ae^-kt. If the temperature of the drink is 15 degrees Celsius after 20 minutes, what will it be after 40 minutes?

I understand that the t stands for time, but I am unsure of the significance of the A