Problem-Solving and Data AnalysisHardConditional probability from counts
Problem-Solving and Data Analysis practice question
Of the 50 plants in a study, 30 were watered daily and 20 were watered weekly. Of the daily-watered plants, 24 bloomed. If a plant that was watered daily is selected at random, what is the probability that it bloomed?
- A. 4/5Correct
- B. 12/25
- C. 3/5
- D. 1/5
Answer: A. 4/5
The condition 'watered daily' restricts attention to those 30 plants. 24 of them bloomed, so the probability is 24/30 = 4/5.
Why the other answers are wrong
- B. 12/25
- 12/25 is 24/50 — the daily-watered plants that bloomed out of ALL 50 plants. The selection was made among the 30 daily-watered ones only.
- C. 3/5
- 3/5 is 30/50, the fraction of plants that were watered daily. That's not what was asked.
- D. 1/5
- 1/5 is 6/30, the daily-watered plants that did NOT bloom. The question asks about blooming.
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