Classical Probability Model

Hi, well I'm totally lost with this subject, and this is an easy question but I just don't understand ..

Question: A fair coint is flipped n = 100 times. What is the probability to have exactly k = 50 heads? What is the probability to have exactly n/2 heads when n is even?

Answer:

Use the Classical Probability Model: with

*<I don't understand why it is * *. Why isn't it just 100? >*

So

*<Again, I don't get why we're using the combination formula? Why isn't it just * *...????>*

*Can someone please explain this to me?*