Answer : We know that,
Probability of occurrence of an event
By permutation and combination, total no.of ways to pick r objects from given n objects is nCr
Now, total no.of ways to pick a ball from 20 balls is 20c1 = 20
Our desired output is to pick a white ball. So, no.of ways to pick a white ball from 7 white balls(because the white ball can be picked from only white balls) is 7c1 = 7
Therefore, the probability of picking a white ball Conclusion: Probability of picking a white ball from 9 red,
7 white and 4 black balls is
7/20