You can use the addition rule. For example, if you are asked the probability of getting at least two heads in three tosses of a fair coin, you're actually being asked to calculate the probability you will get two heads or three heads. You can't get both exactly two heads and exactly three heads, so the events are mutually exclusive. You add the chances of getting exactly two heads and the chance of getting exactly three heads to find the required probability. A binomial coefficient gives the number of ways a thing can happen when making several draws from a box with two types of tickets.
Factorial notation is a shorthand way for writing long products of numbers.
This is very useful if you are trying to write down the product of the first 100 positive numbers. A binomial coefficient is a fraction with one factorial in the numerator, and two in the denominator. There are two factorials in the denominator. They represent the number of tickets of each type that you want to get from the number of draws. The binomial coefficient is sometimes called the number of combinations .