Unit 7 Probability and Counting

Probability and Counting

Probability is the study of how likely an event is to happen. We can use many different methods to do this and it applies to a variety of real world situations. A game of poker, being struck by lightning, or being eaten by a shark are all events that we can calculate the likelihood of happening. This page is going to cover all of the material gone over in the probability unit to prepare you for the final exam.

Diagrams and Charts

There are three ways to visually represent a probability problem: Tree diagrams, area diagrams, and a decision chart.

Tree Diagram

Here we see a tree diagram detailing a one and one free throw for a 70% free throw shooter. We take the first free throw and write out the chances of each result. There is a 70% chance he makes it, 30% chance he misses it. If he makes it, he earns a second free throw, in which he has the same chances of making and missing it. To calculate the probabilities of the three outcomes, we multiply the percentages of a make or miss. For two points, we multiply the 70% make percentage twice to get a .49, or 49% chance of making both free throws. The pattern follows for the other scenarios.

Area Diagram

The Area Diagram follows a similar pattern as the tree diagram, the only difference being it is drawn differently. We are using the same situation for this diagram. The first free throw is represented on the left side and the second is written on the top. To calculate the chances of an event happening, simply multiply the percentages that align with a certain box. To figure out the chance of making one free throw and missing the next, multiply .7 by .3 to get .21, or 21% chance of that happening.

Decision Chart

The Decision Chart requires a different situation than the one and one free throw. Each charts have their own limitations, which are discussed below. The scenario for our decision chart is picking 5 random playing cards from a group of 25. Because no playing card is the same, when you take one away, you have one less option to choose from. The chart shows how many choices you have as you take the cards. When picking your 4th card, you have 22 options to choose from. To calculate the number of possibilities, multiply the 5 amounts of choices together. This rule applies to a decision chart with 2 decisions or X decisions. After calculating the number of possibilities, you can now calculate the probability of a scenario. We have named 5 random cards (assuming they are apart of the 25) and written them down. That is 1 possible scenario out of 6,375,600. To calculate the chance of that sequence, divide 1 by 6,375,600.

Combinations

Permutations

Applications