If the radiation level is 100%, it would take 2.4 days to take the radiation level to 50%. However, the situation implies that the radiation level is 25% only, so we need to take 25% from 2.4 days to know the timeframe for a room to be emptied from the radioactive gas. 2.4 days multiplied by 0.25 will give a result of 0.6 day - the timeframe needed to empty the room out of the radioactive gas.
