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Probability

Question
CBSEENMA12033748

There are two bags I and II. Bag I contains 2 white and 4 red balls and Bag II contains 5 white and 3 red balls. One ball is drawn at random from one of the bags and is found to be red. Find the probability that it was drawn from bag II.

Solution

Bag I

Bag II

Number of white balls = 2

Number of white balls = 5

Number of red balls = 4

Number of red balls = 3

Total number of balls = 6

Total number of balls = 8

Let E1 and E2 be the events of the ball being drawn from bag I and bag II respectively and E be the event that the drawn ball is red.
Since both the bags are equally likely to be selected
therefore space space space straight P left parenthesis straight E space left enclose straight E subscript 1 end enclose right parenthesis space equals space Probability space of space drawing space straight a space red space ball space from space bag space straight I space equals space 4 over 6 space equals 2 over 3
          straight P left parenthesis straight E space left enclose space straight E subscript 2 end enclose right parenthesis space equals space Probability space of space drawing space straight a space red space ball space from space bag space II space equals space 3 over 8
             straight P left parenthesis the space red space ball space is space drawn space from space bag space II right parenthesis space equals space straight P left parenthesis straight E subscript 2 space left enclose space straight E end enclose right parenthesis
                                equals space fraction numerator straight P left parenthesis straight E subscript 2 right parenthesis. space space straight P left parenthesis straight E subscript 2 space left enclose space straight E end enclose right parenthesis over denominator straight P left parenthesis straight E subscript 1 right parenthesis. space straight P left parenthesis straight E subscript 1 space left enclose straight E right parenthesis space plus space straight P left parenthesis straight E subscript 2 right parenthesis. space straight P left parenthesis straight E subscript 2 space left enclose straight E right parenthesis end enclose end fraction
equals space fraction numerator begin display style 1 half cross times 3 over 8 end style over denominator begin display style 1 half cross times plus cross times 2 over 3 1 half 3 over 8 end style end fraction space equals fraction numerator begin display style 3 over 8 end style over denominator begin display style 2 over 3 plus 3 over 8 end style end fraction space equals fraction numerator begin display style 3 over 8 end style over denominator begin display style fraction numerator 16 plus 9 over denominator 24 end fraction end style end fraction space equals fraction numerator begin display style 3 over 8 end style over denominator begin display style 25 over 24 end style end fraction
equals space 3 over 8 cross times 24 over 25 space equals 9 over 25
      

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