Learning Resources

Focus A: Probability

Test yourself (solution)

In a class of 20 students, one is to be chosen to receive a free lunch at a local restaurant. Two students are not interested in going and the rest put their names on a slip of paper, place it in a bag, and have the teacher draw a name. If you are one of the students in the class interested in going:

1. What is the probability your name will be picked?
2. What is the probability your name will not be picked?
3. Why is this an example of theoretical probability?
4. What are the odds your name will be picked?

1. The total number of possible outcomes (slips of paper) is 18 since there are 2 students who do not put their name in the bag. Only one of the the
outcomes favor you (the slip with your name on it). Thus:

2. This is the complement of the first event. We can calculate it two ways: by counting or by using the formula for the complement. Using the latter gives:

3. This is theoretical probability because we actually counted the total number of slips of paper in the bag and the total number of slips that contained your name. It was not done by taking several experimental trials and recording the results.

4. The total outcomes favoring you is 0ne (the slip with your name on it). The total outcomes that do not favor you are 17 (the slips without your name). Thus: