Pipe Music
I am learning to add and subtract decimals.
e.g. 0.3, 0.24, 0.18 etc.
In this lesson we are talking about wholes (like the rod below).
We are also talking about
tenths (0.1)
We are also talking about hundredths
(0.01).
They’re called tenths because it
takes ten tenths to make 1 whole.
You can see it takes ten
hundredths to make 1 tenth
That means it takes 100 hundredths to make one whole (that’s why
we call them hundredths).
You can see below that 5 tenths make a half (0.5).
You can also see that 5 tenths (0.5)
is the same as 50 hundredths.
1
2
It was easy to work out that
of 1 whole equals 5 tenths. Why is it more
difficult to work our what 14 of 1 whole equals?
You can see from the diagram below that 2 tenths (0.2) is less than
1
4
.
You can also see than 3 tenths (0.3) is more than
So to show
1
4
we are going to have to use tenths and hundredths.
1
4
.
You can see that 25 hundredths is the same as
That’s the same as 2 tenths (0.2) ….
Plus 5 hundredths (0.05)
0.2 + 0.05 = 0.25
1
4
.
Steve has 0.3 of a chocolate bar and his brother has 0.04 of a chocolate
bar. How much do they have in total?
Let’s try and make this with materials.
0.34
0.3
0.04
We can also use place value to work out this question.
You can see we have 3 tenths plus 4 hundredths.
0.3 + 0.04 = 0.34
Sue has 1.7 chocolate bars and Alice has 0.9 chocolate bars. How much
chocolate do they have?
Let’s try and make this with materials.
1.7
0.9
1.7 + 0.9 = 2.6
We can also work this out in our head by changing 0.9 to a whole
number. We do this by moving 1 tenth from the 1.7 to the 0.9
1.7 + 0.9 = ?
This side gets
-0.1
smaller by 1 tenth.
+0.1
2.6
1.6 + 1.0 = …..
This side gets
bigger by 1 tenth.
Now let’s try a subtraction question.
Tim has 2.5 chocolate bars and eats 0.9 of a chocolate bar. How much
does he have left?
Let’s try this one on a number line.
- 0.4
1.6
- 0.5
2.0
We need to subtract 0.9. First let’s subtract 0.5
Now let’s subtract the remaining 0.4
2.5 – 0.9 = 1.6
2.5
We could also use equal addition to solve this problem. Let’s look at it
again.
Tim has 2.5 chocolate bars and eats 0.9 of a chocolate bar. How much
does he have left?
This side must also get
bigger by 0.1 to keep the
distance between the two
numbers the same.
2.5 - 0.9 = ?
+0.1
This side gets bigger by
+0.1 1 tenth so it’s now a
whole number.
2.6 + 1.0 = 1.6
…..
Now try these questions on your own.
Click again when you have worked out all your answers.
1) 1.4 + 0.8 =
2.2
6) 2.8 – 0.6 =
2.2
2) 2.3 + 0.02 =
2.32
7) 3.4 – 0.9 =
2.5
3) 1.4 + 0.12 =
1.52
8) 5.77 – 0.03 = 5.74
4) 2.31 + 0.9 =
3.21
9) 1.2 – 0.8 =
5) 1.5 + 2.6 =
4.1
0.4
10) 3.62 – 0.9 = 2.72
Need some more practise? Try Pg28 & 29 of Teacher Tools Fractions,
Decimals and Percentages Book Numeracy resources