I am confused why the answer used the combination to be multiplied by the probability but I will try to go through the whole steps with my explanation and would appreciate your help in checking my understanding here. Regards
Q) 5 students are in a class. What is the probability that exactly 2 have the same birthday?
A) First of all we can find the permutation (p1) for the all the outcomes of having exactly 2 with the same birthday and that is 365x1x364x363x362
Then to find the permutation (p2) representing all the possible outcomes which is the sample space 365^5
Now the probability is (p1/p2 = p) but this probability represents having exactly 2 students with the same birthday and knowing which students they are when selecting. But we are going to select randomly and do not know which students are having the same birthday. We can have 5C2 (=10 different combinations) so px10 will give us the probability for selecting at random and still having 2 with the same birthday.
That is my understanding to this and your thoughts are welcome.
Many thanks in advance.