The temperature of a glass of milk at 90C decreases to 25C for 5 minutes.

a) Assuming that the rate of temperature decrease is proportional to the difference of current

milk temperature and constant air temperature Ta = 20C, derive the differential equation

describing milk temperature T(t) as a function of time t.

b) Solve the differential equation and

c) Find at what time milk temperature decrease up to 40C? Present your answer in minutes up to four significant figures.