An urn contains 9 red, 7 white, and 4 black balls.

We know that,

Probability of occurrence of an event

$=\frac{\text { Total no.of Desired outcomes }}{\text { Total no.of outcomes }}$

By permutation and combination, total no.of ways to pick $r$ objects from given $n$ objects is ${ }^{n} \mathrm{C}_{r}$

Now, total no.of ways to pick a ball from 20 balls is ${ }^{20} \mathrm{C}_{1}=20$

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 ${ }^{16} \mathrm{C}_{1}=16$

Therefore, the probability of picking a white or red ball $=\frac{16}{20}$ $=\frac{4}{5}$

Conclusion: Probability of picking a white or red ball from 9 red, 7 white, and 4 black balls is $\frac{4}{5}$