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A bag contains 8 balls, whose colours are either white or black. 4 balls are drawn at random without replacement and it was found that 2 balls are white and other 2 balls are black. The probability that the bag contains equal number of white and black balls is :

1. 2 5
2. 1 7
3. 1 5
4. 2 7

Correct answer: 2 7

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