(d-15)^2
T= _______ + 20
300
The average temperature around the Tropic of Cancer for a particular day in January can be modeled using the equation above, where T is the temperature in degrees Celsius and d is the number of days after January 1 (on January 1, d=0). According to this model, what is the lowest average temperature, in degrees Celsius, for a day in January, which has 31 days?