Placeholders with Power

A variable is a letter that represents an unknown number. When you say "I'll be there in x minutes," x is a variable. Algebra formalizes this: instead of "some number plus 3 equals 10," write x + 3 = 10. The letter is a placeholder waiting to be filled.

Variables are not just unknowns — they can represent quantities that change. In the formula d = rt (distance equals rate times time), r and t are variables because they can take many values. One equation describes every trip at every speed for every duration. Al-Khwarizmi's 9th-century book gave us both the word "algebra" and the systematic method of solving for unknowns.