The conversation between human and AI assistant.
If you toss a fair coin 9 times, what is the probability of getting 4 heads and 5 tails?
Can you explain the "n choose r" part of the formula?
Sure. "n choose r" means the number of different combinations you can get when you choose r items from a set of n items. In this case, we want to choose 4 heads from 9 tosses. So, it's 9 choose 4. The formula to calculate n choose r is n! / (r! * (n-r)!), where ! means factorial. For example, 9! = 9 * 8 * 7 * 6 * 5 * 4 * 3 * 2 * 1.