elektraa wrote:

I am not very good at probabitliuy. Can someone explain to me in detail how they get the 4 exclusion?

Can someone explain in detail how the answer is arrived?

Models: (Shortest) 1,2,3,4,5,6,7 (Tallest)

First collect all possible combinations of 5 models from 7 models.

7C5 = 21. (Any 5 selected can be made to stand in order of height)

Now it is required that 4th & 6th tallest model should not be together.

So, find out combnation of 5 models in which "5" is not there, because only when "5" is not there, 4 & 6 will be together, got it?

Let's see, how many combinations of 5 models are there (in 7C5) such that 4 & 6 is present but 5 is not present.

Consider 4 & 6 selected (i.e. 2 models are selected & remaining 3 are to be selected), so we are left with 1,2,3,5,7. We don't want "5", so we are now left with 1,2,3,7.

Now, in how many ways can we select remaining 3 models from these 4 models? 4C3 = 4.

Hence, total = 7C5 -4 = 21-4 = 17