If I give unlimited time to write an exam, students exam ti

If I give unlimited time to write an exam, students’ exam times are normally distributed with a mean of 3 hours and a standard deviation of 1 hour.  Students are divided into four rooms.  Room 1 will hold the slowest 10% of the class, Room 2 will hold the next slowest 25% of the class.  Room 3 will hold the next slowest 45% of the class.  Room 4 will hold the fastest 20% of the class.  What is the range of completion times for each room.