What does a variable mean in an addition expression

In addition or subtraction, a variable represents an unknown number.

Example (s):

1. 20 - x = 17; the variable is 'x'

2. 5 + d = 25; the variable is 'd'

Therefore, you can use any letter to represent a variable. If you don't know what the unknown number is, just pretend that the variable is either a 1 or 0.