I'll give this one a shot
The probabilty that 2 planes use runway 1 is (1/2)^2
The probability that 1 plane uses runway 2 is (1/8)^1
The probability of three planes u8se runway 3 is (3/8)^3
And we have 6! arrangements of these probabilities....but some are indistinguishable from others. So, the number of distinguishable arrangements is 6! / [ 2! * 3!] = 60
So the probability is .... 60*(1/2)^2*(1/8)^1*(3/8)*3 = 0.098876953125 = about 9.8%
