Question

A bag contains 15 white and some black balls. If the probability of drawing a black ball from the bag is thrice that of drawing a white ball, find the number of black balls in the bag.

A 20 Correct Answer Incorrect Answer
B 45 Correct Answer Incorrect Answer
C 10 Correct Answer Incorrect Answer
D 50 Correct Answer Incorrect Answer
E None of these Correct Answer Incorrect Answer

Solution

Number of white balls = 15
Let x be the number of black balls.
Total number of balls in the bag = 15 + x
Also, the probability of drawing a black ball from the bag is thrice that of drawing a white ball.
⇒ x/(15 + x) = 3[15/(15 + x)]
⇒ x = 3 × 15 = 45
Hence, the number of black balls in the bag = 45

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