Estimating

Estimating

​

Why use estimation?

• It shows that you understand a process, and can come up with a realistic answer.

• It's convenient in real life - do you want to be getting your calculator out at the supermarket?

• Sometimes, a general estimation will do - if you're trying to work out how many potatoes you'll need for a dinner party for example.

• It's a life skill - being able to estimate how long a journey will take, for example.

How do I estimate?

• By rounding to the nearest ten - so 196 x 11 would become 200 x 10

• By rounding to decimal places - so 11.8 - 15.4 would become 12 x 15

• By truncating (a fancy word for making something shorter) - so 5.87641 would become 5.8

How do I estimate (continued)

• Sometimes you need to find a lower estimate and a higher estimate.

• For example, if I earned £1384 a month and wanted to work out my salary, I might work out £1300 x 12 (harder but more accurate) AND £1500 x 10 (easier but less accurate)

• This gives me a salary of between £15,600 and £15,000

• Split the difference and call it £15,300!

Multiple Choice

Which sum would you use to estimate 36 x 21?

• 30 x 20

• 40 x 20

• 40 x 30

• 30 x 30

Open Ended

Aquil earns £288 per week. He needs to work out his annual salary. Use estimation to work out his annual salary.

Open Ended

Peter sold 89 pairs of scissors for £24 each. They cost him £11. Use estimation to work out how much profit he made.

Open Ended

A coin weighs 12g. Sam has a bag of coins that weighs 983g. Use estimation to work out how many coins he has.

Open Ended

Trudi earns £1628 per month. She needs to work out her annual salary. Use estimation to work out her annual salary.

This is where you can use a higher and a lower estimate and find the mid-point between the 2!

Feeling confident?

Let's look at some exam questions....

Multiple Choice

Shanika sold 378 handbags for £58 each. She paid £32 each for them. Use estimation to work out how much profit she made.

• £18,000

• £20,000

• £22,200

• £24,000

Open Ended

A wall has an area of 123m2 A tin of paint will cover 14m2 of wall. Hattie needs to work out how many tins of paint she will need to buy. Use estimation to work out how many tins she needs.

Type out the sums you did you work it out!

Open Ended

A farmer bought 3179kg of seeds at a cost of £17 per kilogram. Use estimation to work out the total cost of the seeds.

Open Ended

A machine operator makes 74 bolts every hour. She works 39 hours per week for 48 weeks of the year. She will get a bonus if she makes more than 130,000 bolts in a year.

Use estimation to work out if she will receive a bonus this year.

You must show the sums you did to work it out.

Open Ended

Aaron is building a brick wall. The wall will need 1025 bricks. The weight of each brick is 2.89kg. Aaron will pay a delivery charge if the total of bricks is over 2500kg.

Use estimation to work out if you think Aaron will need to pay the delivery charge. You must show your working out.

Great Work!