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Conditional Probability & Bayes' Theorem · Question 96 of 125

The ball drawn from bag II is red.

Question 96

HardOriginal question

Bag I has 3 red and 2 black balls; bag II has 2 red and 3 black balls. One ball is drawn at random from bag I and put into bag II without its colour being seen. Then a ball is drawn at random from bag II.

The ball drawn from bag II is red. What is the probability that the ball moved was red?

Options for question 96

Show the answer and explanation
CorrectNot quiteAnswer D

Correct answer: D

Explanation

  1. Let E1E_1 be that the ball moved is red and RR that the ball drawn is red: P(E1) P(R∣E1)=35×12=930P(E_1)\,P(R \mid E_1) = \frac{3}{5} \times \frac{1}{2} = \frac{9}{30}, and P(R)=1330P(R) = \frac{13}{30}.

  2. P(E1∣R)=9/3013/30=913P(E_1 \mid R) = \frac{9/30}{13/30} = \frac{9}{13}.

Worth noting

35\frac{3}{5} is the chance that the ball moved was red before the drawn ball is seen; a red draw makes it likelier.

Concept
Bayes' theorem
Typical time
150 s
Your time
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The rule this testsThe theorem of total probability
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Ten questions from Conditional Probability & Bayes' Theorem, at your own pace.

Practise this chapter →

Ten questions from Conditional Probability & Bayes' Theorem, at your own pace.