Deepak ranks twenty from top in a class of $50$ students. What is his rank from the bottom?

We can find the rank from the bottom using the standard linear ranking formula:
$$\text{Total number of students}=\text{Rank from Top}+\text{Rank from Bottom}-1$$

Given:

* $\text{Total}=50$
* $\text{Rank from Top}=20$

Substitute the values into the equation:

$$50=20+\text{Rank from Bottom}-1$$

$$50=19+\text{Rank from Bottom}$$

$$\text{Rank from Bottom}=50-19=31$$

Deepak's rank from the bottom is $31^{\text{st}}$.