DASP MATHS: MENTAL CALCULATIONS
Skill:
To find 50%, 25% or 10% of
amounts.
To find 10% or multiples of
10% of quantities and
amounts.
What does this mean?
Your child should be able to quickly calculate 50%, 25%
and 10% of amounts by using their knowledge of
equivalence between fractions and percentages. They
should then use these facts to find percentages such
as 20% or 75% or 5% of amounts.
Examples:
 Find 50% of £20, what is 25%? How did you calculate
this?
 Why is 10% of 40kg = 4kg? How do I find 20%? What
about 80? 5%?
To do the above you also need to be able to do:
 Divide these numbers by 10: 50, 70, 120, 560, 128, 34.3
 Halve these amounts: 30kg, 400m, 75litres
 Move these digits on a place value chart to show what happens
when you divide by 10.
TTh Th H T U t
3
4
5 3 0
Prior learning needed:





Children will be aware of multiples of 10, the 10 times table and
associated division facts.
They will have had practical experience of multiplication and
division by sharing and grouping.
They will know that a percentage is a fraction where the
denominator is 100.
They can find fractions of an amount by dividing by the
denominator and multiplying by the numerator.
They will have knowledge of equivalence between fractions and
percentages.
Key Vocabulary:
Place value
Denominator
Numerator
Percent (%)
Equivalence
Tenth
Hundredth
Decimal point
Half
Quarter
Share
Divide
How to support your child in learning this skill:
Calculating percentages using fractions
Ask if pupils can calculate 50%, 25%, 10% and 1% of an amount in their heads using fractions. Use the
examples below to explain how. To find 75% we add 50% amount to the 25% amount.
Using known percentages
Discuss how to work out the percentage below by using other percentages that are easier. Find 20% by
doubling 10%, 60% by adding 50% and 10% (e.g. 60% of 450 ml = 225 ml + 45 ml = 270 ml) or by
multiplying 10% by 6 (e.g. 60% of 450 ml = 45 ml × 6 = 270 ml), and 99% by subtracting 1% from the total
amount.
Resources to use/ Games to Play:
Percentage spider diagram
Create a percentage spider diagram like the one below by first finding out what 10%, 50% and 1%
(extension) are first and then calculating multiples of these amounts to find the required percentage.
£2.·70
0.54
£______
20.25
£______
1.35
£______
£0.·27
2%
10%
1%
75%
£27
50%
5%
20%
5.40
£______
13.50
£______
25%
6.75
£______
http://www.bbc.co.uk/schools/ks2bitesize/maths/number/percentages/play.shtml
This game looks at percentages.
http://www.mathsisfun.com/percentage-menu.html
Learn about percentages.
http://www.sumdog.com/
Play level 9 or 10 on a variety of games that involve many mental strategies.
Next steps:
Encourage your child to work out other real life percentages such as Blue Cross discounts (20%),
sales (from clothes catalogues) or increases (savings).
Tell your child that 17∙5% is the amount of VAT (short for Value Added Tax) added onto many of the
goods we buy. Work out 10%, halve it to find 5%, and halve this answer to find 2∙5%. Add these three
answers to give 17∙5%.