Download Decimals[1] - Beith Primary School

Decimals
Your child will use decimals in many aspects of life. The most obvious is when
they are using money but they will also use decimals when they are:
measuring height – How tall are you? 1.2m
measuring length –
How far did the athlete jump? 4.73m
measuring time –
How quickly did the racing car complete a circuit?
1 min. 23.706 seconds
converting currency - How many Euros will I get for £100? 116.15
To understand decimals your child should first understand place value.
When we write numbers, the place of each number is important.
In the number 573
the 3 is in the Units position, meaning just 3,
the 7 is in the Tens position, meaning 7 tens or seventy and
the 5 is in the Hundreds position, meaning 5 hundreds or five hundred.
As we move to the left, each position is 10 times bigger – from Units, to Tens, to
Hundreds and as we move to the right, each position is ten times smaller – from
Hundreds, to Tens, to Units.
When we go on past Units, we have tenths. We must use a decimal point, so we
know exactly where the Units position is.
The number 573.8
is five hundred and seventy three and eight tenths or
five hundred and seventy three point eight.
Now we can continue with smaller and smaller numbers, from tenths to
hundredths, to thousandths and so on. As we move further right, every position
gets ten times smaller.
Your child should understand that the digits to the left of the decimal point are
whole numbers and the numbers to the right of the point are decimal fractions.
Adding and Subtracting Decimals
When your child is asked to add or subtract decimals, the most important thing
to do is line up the decimal points.
Add 25.7 to 89.1
Line up the decimals:
2 5 . 7
+ 8 9 . 1
Then add normally, remembering to put in the decimal point.
Subtract 36.4 from 98.7
Line up the decimals:
9 8 . 7
+ 3 6 . 4
Then subtract normally, remembering to put in the decimal point.
If the numbers being added have different amounts of digits you can pad
the number with zeroes.
Add 52.97 to 36.4
5 2 . 9 7
Line up the decimals:
+ 3 6 . 4 0
Pad with a zero
Then add normally, remembering to put in the decimal point.
When subtracting use the same rule.
Subtract 29.6 from 82
8 2 . 0
Pad with a zero
- 2 9 . 6
Line up the decimals
Then subtract normally, remembering to put in the decimal point.
It is important to remember that you must not put zeroes in between
digits which are already there.
Subtract 35.4 from 59.6
5 9 . 0 6
- 3 5 . 4 0
The value of this number has changed.
Multiplying and Dividing Decimals
When multiplying by 10, your child will be taught to move the digits one
place to the left. They will not be taught to add a zero. Although this
works when multiplying whole numbers, it does not work when multiplying
decimals. When multiplying by 100, they will be taught to move the digits
two places to the left.
When dividing by 10, they should move the digits one place to the right and
when dividing by 100 they should move the digits two places to the right.
To help your child remember - the number of zeroes shows the number of
places to be moved.
multiply/divide by 10 – the 10 has one zero so move the digits one place
multiply/divide by 100 – the 100 has two zeroes so move the digits two
places.
When you child is taught to multiply a decimal by a whole number, they will
set it out like this:
2 7 . 6
X 1 1
2
5 5 . 2
When multiplying we start from the right, the tenths,
and work to the left.
When dividing a decimal, your child will be taught to set out the calculation
horizontally.
1 .
3 3 .
2
6
The decimal point always lines up in the question and
answer – one above the other. When dividing, start at the
left and work to the right.
An important thing to remember is that when your child is writing money
they should not write the £ and p together.
74p is not £0.74p.
It is either 74p or £0.74
Here are some activities to try:
1. When you are out shopping, ask your child to tell you the total price of
the items you are buying.
2. Let your child measure the heights of family members. Who is tallest?
How much taller are you than your brother?
3. If going on holiday let your child work out how many Euros etc. they can
get for their money.
4. Discuss the timings of sporting events on TV. Eg. motor racing, athletics.
Who is quickest? How much slower is that driver?
Some useful websites:
http://www.bbc.co.uk/skillswise/numbers/fractiondecimalpercentage/compa
ring/comparingall3/
http://www.bbc.co.uk/skillswise/numbers/fractiondecimalpercentage/decim
als/introductiontodecimals/
http://www.bbc.co.uk/skillswise/numbers/fractiondecimalpercentage/compa
ring/fractionsdecimals/game.shtml
http://www.ictgames.com/equivalence.html