
What is the meaning 4|5 4|3 2|1
I think it (4 boys x (2 girls x 3 boys) x (4 boys x(2 girl ... I wish that I understand so well and thank you.

What is the meaning 4|5 4|3 2|1
I think it (4 boys x (2 girls x 3 boys) x (4 boys x(2 girl ... I wish that I understand so well and thank you.
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The picture is supposed to help us count the number of ways one can have a boy at the left end.
There are $4$ ways to choose that boy. For each such choice, there are $5$ ways to choose the next person, then $4$ ways to choose the next, then $3$ ways to choose the next, and so on, for a total of $(4)(5)(4)(3)(2)(1)=480$.
Similarly, there are $(6)(5)(4)(3)(2)(1)=720$ ways to arrange $6$ people in a row, with no restriction. That is how the book got the probability that a boy is at the left end is $\frac{480}{720}$.
Remark: Note that there is an easier way to solve the probability problem. For the person on the left is equally likely to be any of the $6$ people. Since there are $4$ boys, the probability it is a boy is $\frac{4}{6}$.