6 At a petrol station cars arrive independently and at random times at constant average rates of 8 cars per hour travelling east and 5 cars per hour travelling west.
- Find the probability that, in a quarter-hour period,
(a) one or more cars travelling east and one or more cars travelling west will arrive,
(b) a total of 2 or more cars will arrive. - Find the approximate probability that, in a 12 -hour period, a total of more than 175 cars will arrive.