Answer:
3/23 or 0.1304 or 13%
Explanation:
Given 3 students have a brother and sister, and 20 students have only a brother, you want the probability that a student who has a brother also has a sister.
Conditional probability
The conditional probability formula is ...
P(A | B) = P(A&B)/P(B)
Here, this means ...
P(has a sister | has a brother) = P(has both)/P(has a brother)
= 3/(3 +20)
= 3/23 ≈ 0.1304 ≈ 13% . . . . . probability of a sister if has a brother
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