1. Combinations A combination is similar to a Permutation, with the exception that order does not matter. Recall that n P r refers to the number of ways of arranging n objects in groups of r . For example, the number of possible pairs of letters in the alphabet: However, permutations treat AB as different than BA…. What if we don’t care about the order? This is a combination. In the example above, there would be half as many possibilities…

3. Twenty points are arranged on a circle. How many segments are there with these points as their endpoints? How many triangles? Quadrilaterals?

4. Three students from this class are to be picked to be interviewed by the Inquirer. What is the probability that neither you nor your best friend in the class will be picked?