Answer as a fraction = 1/68880 which is exact
Answer in decimal form = 0.0000145 which is approximate
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Explanation:
Let's call these three cities A, B, and C
There is only one order ABC out of 42*41*40 = 68,880 different possible three city routes to pick from.
So the probability is 1/68880
If you want the answer in decimal form, then use your calculator to find
1/68880 = 0.0000145 approximately