The phrase a number means an arbitrary variable. We can pick any letter a-z except for i and e. Let's choose x. Twice a number means we multiply x by 2: 2x Subtract four: 2x - 4 The word is means equal to. We set 2x - 4 equal to 20 for our algebraic expression: 2x - 4 = 20 If the problem asks you to solve for x: we plug this equation into our calculator and get x = 12
