Let $\frac{dT}{dt} \propto (T_R - T)$, 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)