#### Answer

In the below expression 21 + d, 21 represents a known numerical value. The value of d, however, can be any numerical value as it is not specified.
d may represent any number, therefore, d is the variable.

#### Work Step by Step

A variable is a symbol that represents a number that is unknown.
In the expression 21 + d, d is the only symbol (letter) that represents a number we don't know yet.
Therefore, d is the variable