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Of 755 drivers, 70 had a speeding violation and 25 of those use a phone. What is the probability of phone use given a violation?
AIt is 25/305
BIt is 25/70
CIt is 70/755
DIt is 25/755
Answer & Solution
Correct answer: B. It is 25/70
1. Conditioning on a violation reduces the sample space to the 70 drivers who had one.
2. Of those 70, exactly 25 use a phone while driving.
3. The conditional probability is therefore 25/70.
4. Dividing by 755 instead would give the joint probability and ignore the condition.
_Source: OpenStax Introductory Business Statistics (CC BY 4.0), Ch 3 "Probability Topics", section 3.4 Contingency Tables and Probability Trees_
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