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A probability teaser Suppose (as is roughly correct) that each child born is equally likely to be a boy or a girl and that the genders of successive children are independent. If we let BG mean that the older child is a boy and the younger child is a girl, then each of the combinations BB, BG, GB, and GG has a probability 0.25Ashley and Brianna each have two children.

(a) You know that at least one of Ashley鈥檚 children is a boy. What is the conditional probability that she has two boys?

(b) You know that Brianna鈥檚 older child is a boy. What is the conditional probability that she has two boys?

Short Answer

Expert verified

Part (a) 33.33%

Part (b) P (Two boy At least one boy) = 0.50

Step by step solution

01

Part (a) Step 1. Given Information

At least one of Ashley's children is suffering from the disease.

02

Part (a) Step 2. Concept

Condition probability: P(A|B)=P(AandB)P(B)

03

Part (a) Step 3. Calculation

Use the following formula to calculate the probability of a condition:

P(A|B)=P(AandB)P(B)

P(Twoboy||oldestisboy)=P(Twoboyandoldestisboy)P(oldestisboy)=P(Twoboy)P(oldestisboy)

P(Twoboys)=favourableoutcomespossibleoutcomes=14=0.25

P(Atleastoneboy)=favourableoutcomespossibleoutcomes=34=0.75

As a result, the conditional probability is:

P(Twoboy||Atleastoneboy)=P(Twoboys)P(Atleastoneboy)=0.250.75=130.3333=33.33%

As a result, the chances of her having two sons are 33.33%

04

Part (b) Step 1. Calculation

Condition probability: P(A/B)=P(AandB)P(B)

P(Twoboy||oldestisboy)=P(Twoboyandoldestisboy)P(oldestisboy)=P(Twoboy)P(oldestisboy)

P(Twoboys)=favourableoutcomespossibleoutcomes=14=0.25

P(oldestisboy)=favourableoutcomespossibleoutcomes=24=0.50

As a result, the conditional probability is: P(Twoboy||Atleastoneboy)=P(Twoboys)P(Atleastoneboy)=0.250.50=0.50

As a result, the conditional chance of her having two sons is 0.50

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