On a mild Saturday morning while people are working inside, the furnace keeps the temperature inside the building at 21 deg C. At noon the furnace is...

 On a mild Saturday morning while people are working inside, the furnace keeps the temperature inside the building at 21 deg C. At noon the furnace is turned off, and the people go home. The temperature outside is a constant 12 deg C  for the rest of the afternoon. If the time constant for the building is 3 hr, when will the temperature inside the building reach 16 deg C ? If some windows are left open and the time constant drops to 2 hr, when will the temperature inside reach 16 deg C ?