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There are three identical boxes I, II and III, each containing two balls. In box I, both balls are red, In box II, both balls are blue and box III contains one blue ball and one red ball. A boy randomly chooses a box and takes out a ball at random from it. If the ball is red, then the probability that the other ball in the box is also red colour is:

Solution

✅ Correct Option: 2

By Bayes theorem, P(Box I∣Red)=P(Red∣I)P(I)P(Red)P(\text{Box I} | \text{Red}) = \frac{P(\text{Red}|I) P(I)}{P(\text{Red})}.

P(Red)=13(1)+13(0)+13(12)=12P(\text{Red}) = \frac{1}{3}(1) + \frac{1}{3}(0) + \frac{1}{3}(\frac{1}{2}) = \frac{1}{2}.

P(Box I∣Red)=1/31/2=23P(\text{Box I} | \text{Red}) = \frac{1/3}{1/2} = \frac{2}{3}.

The other ball is red only in Box I.

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