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 red ball. So, no.of ways to pick a red ball from 9 red balls (because the red ball can be picked from only red balls) is ⁹c1 = 9

Therefore, the probability of picking a red ball

Conclusion: Probability of picking a red ball from 9 red,

7 white and 4 black balls is

9/20