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Question

A bag contains 20 white and some black balls. If the probability of drawing a black ball from the bag is 2.5 times that of drawing a white ball, find the number of black balls in the bag.

A 50 Correct Answer Incorrect Answer
B 38 Correct Answer Incorrect Answer
C 36 Correct Answer Incorrect Answer
D 26 Correct Answer Incorrect Answer
E None of these Correct Answer Incorrect Answer

Solution

Number of white balls = 20
Let x be the number of black balls.
Total number of balls in the bag = 20 + x
Also, the probability of drawing a black ball from the bag is 2.5 times that of drawing a white ball.
⇒ x/(20 + x) = 2.5[20/(20 + x)]
⇒ x = 2.5 × 20 = 50
Hence, the number of black balls in the bag = 50

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