In this activity, we'll be learning how to change missing number problems into algebra.

So first... what is **algebra**?

We use **algebra **when we don't know the exact numbers in a calculation, and to do this we use letters to stand for numbers that we are trying to find.

Algebra is like a language with its own rules! Let's look at some of the rules we need to know.

1)* a* stands for a number (we can use any letter)

2) * a + 2* is

**2 more**than the number.

3)* a - 7* is

**7 less**than the number.

4)* 5a* is

**5 times**the number

*(we don't use the × sign in algebra because it looks like the letter x).*

5) * a/4* or

*is a quarter of the number or the*

**¼ a****number divided by 4**.

*(N.B. We don't use the ÷ sign in algebra.)*

**Let's now use our algebra knowledge to work out the following problem:**

I think of a number.

1) I double it and 2) take away 5.

3) I get 21.

Write this using algebra.

Use * n* for the number I think of.

**Solution**

* n *is the number I think of.

1) Double it to get **2n **(remember this means 2 times n)

2) Take away 5 to get** 2n - 5**

3) This equals 21, so we write:

**2n - 5 = 21**