If from twice a number you subtract four, the difference is twenty

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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