Transcript Division by inverse multiplication, repeated subtraction (PPT

Division
Using multiplication and
repeated subtraction
What’s It All About?
You are going to learn:
How to divide by inversing multiplication and using
repeated subtraction.
What skills should you have already?
You need to be able to multiply and subtract two
values using a written method.
Division
Remember that 6 ÷ 3 means “how many threes make 6?”
Division is the INVERSE of Multiplication.
There are different ways of showing division.
6÷3
6
3
The answer is the same in all three cases!
3 6
Using Multiplying to Divide
For multiplying, we can use a grid like this:
2
3
4
5
6
1
2
2
4
3
6
4
8
5 6 7 8 9
10 12 14 16 18
3
6
9
12 15 18 21 24 27
4
5
8 12 16 20 24 28 32 36
10 15 20 25 30 35 40 45
6
12 18 24 30 36 42 48 54
We can now use it to divide as well as multiply.
Example: 20  4 means “How many 4’s make 20?”
The answer is 5.
Your Turn
Remember that 6 ÷ 3 means “how many threes make 6?”
Work out:
1.
16 ÷ 4
2.
30 ÷ 5
3.
30 ÷ 10
4.
45 ÷ 9
5.
72 ÷ 8
6.
21 ÷ 3
7.
21 ÷ 7
Repeated Subtraction 1
Work out £192 ÷ 6
Set up the method:
6
192
60
132
60
72
60
12
12
0
£192  6 = 32
10
10
10
2
32
Multiply 6 by
something easy such
as 10 or 100, then
subtract.
Repeat this until you
have nothing left or
the number that is left
is less than 6.
Stop. Total up the 6’s.
Repeated Subtraction 2
Work out £324 ÷ 12
Set up the method:
12
324
120
204
120
84
60
24
24
0
10
10
5
2
27
£324  12 = 27
Multiply 12 by
something easy such
as 10 or 100, then
subtract.
Repeat this until you
have nothing left or
the number that is left
is less than 12.
Stop. Total up the
12’s.
Repeated Subtraction 3
Work out £3010 ÷ 14
Set up the method:
14
3010
1400
1610
1400
210
140
70
70
0
100
100
10
5
215
£3010  14 = 215
Choosing sensible numbers
reduces the amount of
subtracting you have to do ....
Multiply 14 by something
easy such as 10 or 100,
then subtract.
Repeat this until you have
nothing left or the number
that is left is less than 14.
Stop. Total up the 14’s.
Your Turn
Remember that 6 ÷ 3 means “how many threes make 6?”
Work out:
1.
164 ÷ 4
5.
156 ÷ 12
2.
305 ÷ 5
6.
315 ÷ 15
3.
3040 ÷ 10
7.
3040 ÷ 20
4.
4509 ÷ 9
8.
2814 ÷ 14
End