When we use algebra, we use letters to stand for numbers that we are trying to find.

Algebra is like a language with its own rules.

a stands for a number.

a + 2 is 2 more than the number.

a - 7 is 7 less than the number.

5a is 5 times the number. (N.B. We don't use the × sign in algebra because it looks like the letter x.)

a/4 or ¼a is a quarter of the number or the number divided by 4. (N.B. We don't use the ÷ sign in algebra.)

Example

I think of a number.

I double it and take away 5.

I get 21.

Write this using algebra.

Use n for the number I think of.

Answer

n is the number I think of.

Double it to get 2n.

Take away 5 to get 2n - 5.

This equals 21.

So we write:

2n - 5 = 21