If we flip a coin n-times,(adsbygoogle = window.adsbygoogle || []).push({});

what is the probability of the event $$A= \left \{there \space are \space k \space head \right \}$$.

I should find the number of elements of A,

the book says that is $$\binom{n}{k}$$ but for $$n=3$$ and $$k=2$$, all the possible outcomes are:

$$A= \left \{(h, h,h), (h, h, x), (h, x, h), (x, h, h) \right \}$$, where the position of h or x is the k-flip.

How to find this number?

PS. h is head.

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# Sample spaces having equally likely outcomes

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