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starMay 2012

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The ideas in this multiplication lesson are taken from Division 2 ebook. Only a few examples of each problem type are shown; you should make more problems of each kind for the student.


Multiplying by whole tens - by 10, 20, 30 etc.
Free lesson plan from HomeschoolMath.net

Count by tens:

0, 10, ___, ___, ___, ___, ___, ___, ___, ___, ___, ___, ___, ___, ___, ___, ___, ___,

___, ___, ___, ___, ___, ___, ___, ___, ___, ___, ___, ___, ___, ___, ___, ___.

Based on your counting, fill in parts of the MULTIPLICATION TABLE OF 10.  

9 × 10 = ___

10 × 10 = ___

11 × 10 = ___

12 × 10 = ___

13 × 10 = ___

14 × 10 = ___

15 × 10 = ___

16 × 10 = ___

17 × 10 = ___

18 × 10 = ___

19 × 10 = ___

20 × 10 = ___

21 × 10 = ___

22 × 10 = ___

23 × 10 = ___

24 × 10 = ___

25 × 10 = ___

26 × 10 = ___

27 × 10 = ___

28 × 10 = ___


What pattern do you see?
What is the easy rule for multiplying by ten?

Since multiplication can be done in any order one wishes, or 25 × 10 = 10 × 25, this rule/pattern works in either case.
 

Example problems

1.  Multiply.  

10 × 11 = ___

56 × 10 = ___

10 × 98 = ___

12 × 7 = ___

20 × 10 = ___

18 × 10 = ___

10 × 10 = ___

10 × 0 = ___

9 × 8  = ___

77 × 10 = ___

10 × 5 = ___

97 × 10 = ___

10 × 82 = ___

9 × 9 = ___

2 × 10 = ___

 

Note that 20 = 10 × 2.  We can use that to multiply by 20.  For example,

20 × 11 = 10 × 2 × 11 

Now multiply first 2 × 11 = 22.  Then use the easy rule for multiplying by ten:

10 × 2 × 11 = 10 × 22 = 220.

So we have 20 × 11 = 220.




1.  Multiply by 20.  First break 20 down into 10 × 2.
 

20 × 8 = 10 × 2 × 8
=

10 × 16

= 160
20 × 12 = 10 × __ × __
=

10 × ___

= ____
20 × 9 = ___ × __ × __
=

___ × ___

= ____

 

What is the easy rule for multiplying by 20?

 

Note that 30 = 10 × 3.  We can use that to multiply by 30.  For example,

30 × 9 = 10 × 3 × 9 =  10 × 3 × 9  = 10 × 27 = 270

Note that 50 = 10 × 5.  We can use that to multiply by 50.  For example,

50 × 7 = 10 × 5 × 7 =  10 × 5 × 7  = 10 × 35 = 350

Note that 80 = 10 × 8.  We can use that to multiply by 80.  For example,

80 × 6 = 10 × 8 × 6 =  10 × 8 × 6  = 10 × 48 = 480.



2.  Multiply.  First break 20, 30, 40, etc down into 10 × a number.

50 × 4 = 10 × 5 × 4
=

10 × 20

= 200
70 × 3 = 10 × __ × __
=

10 × ___

= ____
60 × 7 = 10 × __ × __
=

10 × ___

= ____
30 × 8 = 10 × __ × __
=

10 × ___

= ____

 

Write down here the results from the last exercise:
 
50 × 4 =

70 × 3 =

              60 × 7 =

30 × 8 =

             

30 × 5 =

40 × 7 =

             

90 × 4 =

50 × 9 =

 
What is the easy rule for multiplying by whole tens (by 30, 40, 50, 60, 70, 80, or 90)?
How do you use the multiplication tables to help you?

  
3.  Multiply.

a. d.
40 × 5 = ____

8 × 20 = ____

7 × 70 = ____

30 × 4 = ____

3 × 40 = ____

70 × 4 = ____

70 × 11 = ____

  6 × 40 = ____

12 × 30 = ____

  8 × 80 = ____

11 × 40 = ____

4 × 50 = ____



4.  For each multiplication, write two division sentences.

a. 10 × 45 = 450

450 × 10 = 45

450 × 45 = 10

d.   3 × 30 = 
e.  7 × 40 = 

 

 

h.   9 × 60 = 

 
5. Divide.  Think backwards of the multiplication rules you've learned in this lesson.

a.  120 × 10 = c.   100 × 10 = d.   200 × 10 =
i.  120 × 20 = k.   120 × 60 = l.   180 × 60 =
m.  160 × 40 = o.   560 × 70 = p.   420 × 60 =

 
6.  Continue the patterns.

a.

100 × 10 =

200 × 10 =

300 × 10 =

400 × 10 =

 

 

 

d.

100 × 50 =

150 × 50 =

200 × 50 =

250 × 50 =

 

Next lesson: Multiply by hundreds

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