In Your Head 1

1
10%50%25%15%
£16
£22
£70
£5.40
£25.50
£80.40

Copy and complete this table in the back of your exercise book.
No calculator allowed!

 


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Topics: Starter | Percentages

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  • This is a popular starter (as can be seen from the score below) and rightly so. It doesn't have the intrigue of some of our puzzle type starters but this starter provides pupils with practice of a very important life skill as well as a good mental arithmetic work out. A good understanding of percentages will help students develop strategies and short cuts for doing the calculations quickly. This is a starter that is well worth repeating, a different version is available here.

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This starter has scored a mean of 3.2 out of 5 based on 395 votes.


Previous Day | This starter is for 29 September | Next Day

 

Globe of Flags

This activity is suitable for students of mathematics all around the world. Use the button below to change the currency symbol used to make it more relevant to your students. You may wish to choose an unfamiliar currency to extend your students' experience.

Globe of Flags

% - This is the percent symbol.
Percent means 'out of 100'.

Find 50%:

As 50 is half of 100, then 50% means half. To find 50% of a quantity you need to halve (or divide by two). So 50% of 6 is 3.

Find 10%:

As 10 is one tenth of 100, then 10% means 'one tenth of'. To find 10% of a quantity you need to divide it by ten. So 10% of 800 is 80.

Find 25%:

As 25 is one quarter of 100, then 25% means 'one quarter of'. To find 25% of a quantity you need to divide it by four. So 25% of 20 is 5.

Another way of finding 25% of a quantity is first finding 50% then dividing the result by 2.

Find 33⅓%:

As 33⅓ is one third of 100, then 33⅓% means 'one third of'. To find 33⅓% of a quantity you need to divide it by three. So 33⅓% of 30 is 10.

Find 1%:

As 1 is one hundredth of 100, then 1% means 'one hundredth of'. To find 1% of a quantity you need to divide it by 100. So 1% of 800 is 8.

Find other percentages:

Other percentages can be found by combining some of the techniques mentioned above. Here are some examples:

If you need to use a calculator to check your working. See Calculator Workout skill 3.

Commute

Did you know that if you are struggling to mentally work out 24% of 50 you can switch the numbers round and work out 50% of 24 instead. Finding 50% is very easy isn’t it? You will get the same answer.

Finding a percentage of a quantity is an example of a commutative calculation. Not all operations are commutative. Subtraction certainly isn’t as 10 minus one is not the same as one minus ten.

You can use this trick to improve your ability to do this type of calculation quickly if you find the switch makes it easier.

Practise with 12% of 50, 4% of 25 and 75% of 10.

Answers

10%50%25%15%
£16£1.60£8£4£2.40
£22£2.20£11£5.50£3.30
£70£7£35£17.50£10.50
£5.4054p£2.70£1.3581p
£25.50£2.55£12.75£6.38*£3.83*
£80.40£8.04£40.20£20.10£12.06

*These answers have been rounded to the nearest penny

See also In Your Head 2

Note to teacher: Doing this activity once with a class helps students develop strategies. It is only when they do this activity a second time that they will have the opportunity to practise those strategies. That is when the learning is consolidated. Click the link above to find another version of this starter or you can manually change the values in the table above.



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Teacher, do your students have access to computers?
Do they have iPads or Laptops in Lessons?

Whether your students each have a TabletPC, a Surface or a Mac, this activity lends itself to eLearning (Engaged Learning).

Laptops In Lessons

Here a concise URL for a version of this page without the comments.

Transum.org/go/?Start=September29

Here is the URL which will take them to a student version of this activity.

Transum.org/go/?to=Percentages

Student Activity

After Thought

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