Once we are confident with addition, we can use it to solve word problems.

This might include calculating the cost of a number of items in a shop, or working out how much change we should get.

We use addition in many areas of life, sometimes not even realising we are using it.

In this activity, we are going to solve some word problems involving addition and subtraction.

Let's have a look at one.

**I think of a number. I add 20. My answer is 40. What was my starting number?**

Let's put this into a number sentence.

We don't know the starting number so let's put a **?**

We know we need to add **20**

**? + 20 =**

We know we have an answer of **40**

**? + 20 = 40**

We can now subtract the **20 from the 40**** **to find the starting number:

**40 - 20 = 20**

We can check by putting the answer where the** ****? **was.

**20 + 20 = 40**

**The starting number was 20**

Let's have a look at another one:

**I think of a number. I add 25. My answer is 200. What was my starting number?**

Let's put this into a number sentence:

We don't know the starting number so let's put a **?**

We know we need to add **25**

**? + 25 =**

We know we have an answer of **200**

**? + 25 = 200**

We can now **subtract the 25 from 200 **to find the starting number:

**200 - 25 = 175**

We can check by putting the answer where the** ****? **was.

**175 + 25 = 200**

**The starting number was 175**

We must read the question carefully to understand whether we need to add or subtract.

*Writing the inverse operation and putting the numbers back into the word problem can help us check if we are correct!*

Now, why don't you have a go at solving some 'I think of a number' problems?