Answer:
If you are asking about mathematics, a variable would technically be an unknown value or a placeholder for a numerical value; given the situation.
For example, if you were going to calculate the rate of speed of a car. The usual formula for that would be S = D/T (Speed = Distance / Time). If a situation or question gave us Distance & Time, you would need to find speed, right? That's correct, and what we are trying to find would be that variable.
A variable can be expressed in multiple ways: you use the letters x, v, a, s, etc. As long as it is a (singular) letter, it works as a variable.
