In this activity, we'll be rounding numbers (including decimals) to estimate and check the answers to questions.

**Estimating **can help us to decide if the answer to a problem is reasonable.

If our estimate and answer are very different, it is likely we have made an error.

**Recap of rounding**

When rounding, we look at the digit to the right of the number we are rounding to.

If we are rounding to the nearest 100, we look at the **tens **digit.

If we are rounding to the nearest 10, we look at the **ones** digit.

We round up or down depending on the value of this digit.

If this digit is **less than 5,** the number needs to be rounded** down**.

If this digit is** 5 or above**, the number needs to be rounded **up.**

Let's have a look at some examples.

**What is 3.5 rounded to the nearest whole number?**

The digit to the right of the decimal point is in the tenths column and is a **5.**

So 3.5 would be rounded **up** to **4.**

Note: The number rounds to a whole number.

**What is 56 rounded to the nearest 10?**

The digit to the right of the tens digit is in the ones column and is a **6.**

So 56 would be rounded **up** to **60**.

Note: The number rounds up to the next multiple of 10 (up to 60).

**What is 734 rounded to the nearest 100?**

The digit to the right of the hundreds digit is in the tens column and is a **3.**

734 would be rounded** down** to **700** because the tens digit is less than 5.

Note: The number rounds down to 700, not the previous 100 which would be 600.

Let's have a look at a couple of questions using rounding.

The answer to 32 x 47 = 1,504

Round both numbers to the nearest 10 to check if the answer is reasonable.

32 rounds** down** to **30**

47 rounds **up** to **50**

30 x 50

To do this we can do the following:

3 x 5 = 15

30 x 5 = 150

**30 x 50 = 1,500**

We can see that the actual answer (1,504) is very close to the estimate, so we can assume it is probably correct!

Now let's look at a word problem.

Charlie raised £654.20 for charity and Clara raised a further £213.50

Estimate the answer by rounding each amount to the nearest £10 before adding.

£654.20 rounds **down** to** £650**

£213.50 rounds **down **to **£210 **

**£650 + £210 = £860**

The actual answer is £867.70

Can you see how estimating answers helps us to see if our answers are reasonable?

Now it's your turn to have a go at some questions using rounding to estimate answers.