Problem-Solving and Data AnalysisMediumConditional probability from group counts
Problem-Solving and Data Analysis practice question
A café recorded the orders of 120 customers one morning: 70 ordered coffee and 50 ordered tea. Of the coffee drinkers, 42 also bought a pastry. If one of the coffee drinkers is selected at random, what is the probability that this customer bought a pastry?
- A. 3/5Correct
- B. 7/20
- C. 7/12
- D. 2/5
Answer: A. 3/5
The selection is from the 70 coffee drinkers only. 42 of them bought a pastry: 42/70 = 3/5.
Why the other answers are wrong
- B. 7/20
- 7/20 is 42/120 — dividing by ALL customers. The customer is chosen from the coffee drinkers, so divide by 70.
- C. 7/12
- 7/12 is 70/120, the probability that a random customer ordered coffee. That isn't what was asked.
- D. 2/5
- 2/5 is the probability that a coffee drinker did NOT buy a pastry (28/70).
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