Question 1148437
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For finding the probability in this simple problem, there are simpler methods for than shown by tutor @ikleyn.  However, you should understand the method she shows, because it will be needed when the problems are more complicated.<br>
The alternative to her method is to look separately at the probabilities of each person getting in the correct position in the line.<br>
P(Doc is 1st) = 1/7
P(Frank is 2nd) = 1/6
P(Shem is 3rd) = 1/5<br>
P(Doc 1st AND Frank 2nd AND Shem 3rd) = (1/7)(1/6)(1/5) = 1/210<br>