Let dT/dt∝(T_R-T), where T_R and T are temperatures in degree centigrade of a room and thermometer, respectively, and t denotes time in minutes. A thermometer at a reading of 2°C is brought in a room of temperature 40°C. Two minutes later, the thermometer reads 15°C. The time elapsed in minutes when the thermometer reads 39.5°C. is (round off to two decimal places)