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 ⁿC_r

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