Marbles and bags
Problem
Two bags contain different numbers of red and blue marbles. A marble is removed from one of the bags (it is not known from which bag). The marble is blue. What is the probability that it was removed from bag A?
Bag A 2 RED Image
![]() 3 BLUE Image
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Image
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Bag B 3 RED Image
![]() 4 BLUE Image
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Image
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Student Solutions
Assume that removing a ball from bag A or bag
B is equally likely, the probability of choosing a
blue ball from bag A is 3/10 and the probability
of choosing a blue ball from bag B is 4/14 so the
probability of choosing a blue ball is 3/10 + 4/14 = 41/70.
Theoretically in 70 trials, 41 of the outcomes would give a blue
ball and 21 of these would of been drawn from bag
A . Given that the ball drawn was blue the
probability that it came from bag A is 21/41.
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The only correct solution to date came from Matthew (Smithdon High School)