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 or red ball. So, no.of ways to pick a white or red ball from 16 balls(because there are a total of 16 balls which are either red or white) is ¹⁶c1 = 16

Therefore, the probability of picking a white or black ball

Conclusion: Probability of picking a white or black ball from 9 red, 7 white, and 4 black balls is 11/20