An object with initial temperature 150 degree Celsius is placed outside, where the temperature is 35 degree Celsius . Its temperatures at 12:15 and 12:20 are 120 degree Celsius and 90 degree Celsius , respectively.

(a) At what time was the object placed outside?

(b) When will its temperature be 40 degree Celsius .

Present the derivation of the equation using Variation of Parameters. This is an essential part of the exercise.