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

*is a quarter of the number or the number divided by 4.*

**¼a***(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**