Start from zero — the basic math you need before everything else
A variable is a letter (like x or n) standing in for a number we don't know yet, or that can change. It's a placeholder. Algebra is arithmetic where some of the numbers are written as letters.
When you're told what the variable equals, you put that value in its place. This is called substituting. Then you work the arithmetic out. One thing to know: 2x means 2 × x. A number written right next to a letter means "multiply". The × is left out to save space.
A variable is a stand-in for a number you'll fill in later. Planning a party, you might say 'snacks = 2 per child', or s = 2c. The letter c waits for the real number of children; when 6 turn up, you substitute c = 6 and get s = 12 snacks.
▶ What a Variable Is