= P(3 in the first hour) = A1 3! number to arrive in the four hours follows Po(4X) M1 (4A)-4x P(5 arrive in total) = A1 5! attempt to find P(2 arrive in the next three hours) M1 (3A)2-31 = A1 2! use of conditional probability formula M1 13-A (31)-31 x P(3 in the first hour given 5 in total) = 3! 2! == (43)5-41 A1 213! 45 = = 0.0879 A1 512 22. Students sign up at a desk for an activity during the course of an afternoon. The arrival of each 16M.2.hl.TZ1.10 [8 marks] student is independent of the arrival of any other student and the number of students arriving per hour can be modelled as a Poisson distribution with a mean of X. The desk is open for 4 hours. If exactly 5 people arrive to sign up for the activity during that time find the probability that exactly 3 of them arrived during the first hour.