Problem-Solving and Data AnalysisMediumJoint probability from a two-way table
Problem-Solving and Data Analysis practice question
Grade | In the club | Not in the club
Ninth | 15 | 25
Tenth | 20 | 40
If one of the 100 students is selected at random, what is the probability that the student is a tenth grader who is in the club?
- A. 1/5Correct
- B. 1/3
- C. 3/5
- D. 7/20
Answer: A. 1/5
Tenth graders in the club: 20 out of all 100 students. Probability = 20/100 = 1/5.
Why the other answers are wrong
- B. 1/3
- 1/3 is 20/60 — the probability that a TENTH GRADER is in the club, a conditional probability.
- C. 3/5
- 3/5 is 60/100, the probability of being a tenth grader at all.
- D. 7/20
- 7/20 is 35/100, the probability of being in the club regardless of grade.
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