Question 1194074
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The solution from the other tutor is one standard way of answering these two questions.  You should understand it and be able to use it to solve similar problems.<br>
Another standard way of solving these problems is to consider the probabilities of filling the seats one at a time under the given constraints.<br>
(1) girls in first three places...<br>
first place: 7 people to choose from; 4 are girls --> P = 4/7
second place: 6 people to choose from; 3 are girls --> P = 3/6
third place: 5 people to choose from; 2 are girls --> P = 2/5<br>
P(girls in first three places) = (4/7)(3/6)(2/5) = 4/35<br>
(2) alternating...<br>
Obviously a girl must be first, then a boy, and so on.  Without showing in words the probabilities for getting the right gender in the right seat...<br>
P(alternating) = (4/7)(3/6)(3/5)(2/4)(2/3)(1/2)(1/1) = 1/35<br>