1. The problem statement, all variables and given/known data I don't my book to give me the exact wording of the question, but the essence is finding a formula for the probability of n heads after flipping a coin n times. 2. Relevant equations "n choose k" = n! / k!(n-k)! 3. The attempt at a solution Obviously you have a 1:2 chance of flipping n heads. But I'm not sure how to show this, and I think I'm supposed to use the above equation. The number of possible combinations will be 22n; half of those will have n heads.