SE-SKOLORNAS 1972 Problem 3

A steak temperature $5^\circ$ is put into an oven. After $15$ minutes, it has temperature $45^\circ$ . After another $15$ minutes it has temperature $77^\circ$ . The oven is at a constant temperature. The steak changes temperature at a rate proportional to the difference between its temperature and that of the oven. Find the oven temperature.