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, what is the number of black balls in the bag?
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Answer:
Number of white balls =15
Let number of black balls =x
Total number of outcomes =15+x
P
1
= Probability of getting white ball =
15+x
15
P
2
= Probability of getting black ball =
15+x
x
As per question
P
2
=3P
1
⇒
15+x
x
=3×
15+x
15
⇒
15+x
x
=
15+x
45
∴x=45
Hence number of black balls is 45.
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