Mention the predecessor and successor of the number $21$.

Hint: In order to find the successor and predecessor of the given values, first we should know what successor and predecessor is. Successor is a whole number that comes just after the given number, whereas predecessor is a whole number that comes just before the given whole number. And, these values can be obtained by adding or subtracting one from the value given.

Complete step-by-step answer:
We are given with the values $21$.
From the basic definition for successor, we know that the value which comes just after the value is successor of that value, and we can obtain that by adding one to that value.
So, to find the successor of the given number $21$, we would just be adding one to the given value and we get:
$21 = 21 + 1 = 22$
Similarly, for the Predecessor, we know that it is the value that comes just before the given value and that can be obtained by subtracting one from the value.
 So, to find the predecessor of the given number $21$, we would just be subtracting one from the given value and we get:
$21 = 21 - 1 = 20$
Therefore, the predecessor of the number $21$ is $20$ and successor is $22$.

Note: If we are given a negative number, then its successor decreases, because, if we add one number to a negative number it will not move towards left in the number line, it always moves towards right and decrements a number to become positive.
If we were given to find a predecessor for a negative number, the predecessor value increases with a negative sign, as it will move towards the negative side on the number line.