Probability關於要乘整數問題(急!!!!!!!!!

我想問probability幾時要係個分數乘個整數呢？例如呢條數:

Stem(tens) Leaf(units)

1 2 5 5 7 8

2 4 4 4 4 5 6 7 9

3 2 2 2 2 5 5 5 6 8

4 2 3

Find the probability that the mean score of the two students is the same as the mean score of the 24 students. (mean score is 28)

Ans: Let x and y be their marks.

(x+y)/2=28

x+y=56

Possible combinations: (18,38) (24,32), (27,29)

Probability:

2x(1/24x1/23)+2x (4/24x4/23)+2x (1/24x1/23)

=3/46

為什麼要在分數之前乘一個整數2 呢？