Multiplying by Whole Tens and Hundreds
This is a complete lesson with instruction and exercises for fourth grade about multiplying by whole tens and hundreds. The lesson explains the shortcut, and also explains why it works. It contains plenty of exercises for the students, including word problem.
We have studied the SHORTCUTS for multiplying any number by 10, 100, or 1,000:  
To multiply any number by 10, just tag ONE zero on the end. To multiply any number by 100, just tag TWO zeros on the end. To multiply any number by 1,000, just tag THREE zeros on the end. 

10 × 481 = 4,810 
100 × 47 = 4,700  1000 × 578 = 578,000 
Note especially what happens when the number you multiply already ends in a zero or zeros. The rule works the same; you still have to tag the zero or zeros.  
10 × 800 = 8000  100 × 6,600 = 660,000  1000 × 40 = 40,000 
1. Multiply.
a. 10 × 315 = _______ 3,560 × 10 = _______ 35 × 100 = _______ 
b. 100 × 6,200 = _______
10 × 1,200 = _______ 100 × 130 = _______ 
c. 1,000 × 250 = _______ 38 × 1,000 = _______ 10 × 5,000 = _______ 
SHORTCUT for multiplying by 20 or 200 (You can probably guess this one!)  
What is 20 × 14? Imagine the problem without the zero. 
What is 200 × 31? Imagine the problem without the zeros. 
2. Now try it! Multiply by 20 and 200.
a. 20 × 8 = _______ 4 × 20 =_______ 20 × 5 = _______ 
b. 200 × 7 = ________ 5 × 200 = ________ 11 × 200 = ________ 
c. 20 × 12 = _______ 35 × 20 = _______ 200 × 9 = _______ 
d. 20 × 16 = _________ 42 × 200 = ________ 54 × 20 = _________ 
Why does the shortcut work? It is based on the fact that you can multiply in any order. 

When multiplying by 20, we can change the 20 into 10 × 2. For example:
20 × 14 = 10 × 2 × 14 In that problem, first multiply 2 × 14 = 28. Then the problem becomes 10 × 28, which we know is 280. 20 × 14 = 10 × 2 × 14 = 10 × 28 = 280 That's it! 
Let's try the same with 200. For example,
200 × 31 = 100 × 2 × 31 In that problem, first multiply 2 × 31 = 62. The problem now becomes 100 × 62, which is 6,200: 100 × 2 × 31 = 100 × 62 = 6,200 
3. Try it yourself! Fill in.
a. 20 × 7
= ______ × 2 × 7 = 10 × ______ = ________ 
b. 20 × 5
= ______ × 2 × 5 = 10 × ______ = ________ 
c. 200 × 8
= ______ × 2 × 8 = 100 × ______ = ________ 
d. 200 × 25
= ______ × 2 × 25 = 100 × ______ = ________ 
4. Mark's shed measures 20 ft by 15 ft. What is its area? Write a number
sentence.
"A" means area.
A = __________________________________
5.
Write a number sentence, and find
the area of Mark's driveway.
A = __________________________________
6. Mark was told he needed four truckloads of gravel to cover his driveway.
One truckload costs 5 × $20 plus $30 for the delivery. How
much will
it cost him to cover the driveway with gravel?
SHORTCUT for multiplying by whole tens and whole hundreds  
The same principle works if you multiply by whole tens (30, 40, 50, 60, 70, 80, or 90): simply multiply 2, 3, 4, 5, 6, 7, 8, or 9, and then tag a zero to the end result. Similarly, if you multiply by some whole
hundred, FIRST multiply without
those two 

50 × 8 = 400  90 × 11 = 990  300 × 8 = 2,400  12 × 800 = 9,600 
7. Multiply.
a. 40 × 3 = ______ 8 × 20 = ______ 
b. 70 × 6 = _______
50 × 11 = ______ 
c. 80 × 9 = _______
30 × 15 = _______ 
d. 60 × 11 = _______ 12 × 40 = _______ 
e. 200 × 9 = ______ 7 × 400 = ______ 
f. 700 × 6 = ______ 600 × 11 = ______ 
g. 200 × 12 = ______ 15 × 300 = ______ 
h. 3 × 1100 = ______ 8 × 900 = ______ 
i. 11 × 120 = ______ 8 × 300 = ______ 
It even works this way:  
To multiply 40 × 70, simply multiply
4 × 7, and tag two zeros to the result:
40 × 70 = 2,800 
To multiply 600 × 40, simply multiply
6 × 4,
and tag three zeros to the result: 600 × 40 = 24,000 
To multiply 700 × 800, simply multiply 7 × 8, and tag four zeros to the
result.
700 × 800 = 560,000 
8. Multiply.
a. 20 × 90 = _________ 70 × 300 = ________ 
b. 60 × 80 = ________ 30 × 900 = ________ 
c. 400 × 50 = ________ 200 × 200 = ________ 
d. 80 × 800 = ________ 200 × 500 = ________ 
e. 100 × 100 = _______ 40 × 30 = ________ 
f. 800 × 300 = ________ 90 × 1100 = ________ 
Write a number sentence for each question.
9. One hour has ______ minutes.
How many minutes are in 12 hours?
How many minutes are in 24 hours?
10. One hour has ______ minutes, and one minute has ______ seconds.
How many seconds are there in one hour?
11. Ed earns $30 per hour.
a. How much will he earn in a 8hour workday?
b. How much will he earn in a 40hour workweek?
c. How many days will he need to work in order to earn more than $1,000?
12. Find the missing factor. Think “backwards”! How many zeros do you need?
a. _______ × 3 = 360 _______ × 50 = 450 
b. 40 × _______ = 320 5 × ________= 600 
c. ________ × 40 = 400
________ × 2 = 180 
d. _______ × 30 = 4,800 _______ × 200 = 1,800 
e. 40 × ________ = 2,000 6 × _________= 4,200 
f. ______ × 800 = 56,000
_______ × 20 = 12,000 
John wanted to prove that 40 × 70 is indeed 2,800 by 40 × 70 = 4 × 10 × 7 × 10 = 10 × 10 × (4 × 7) = 100 × 28 = 2,800. You do the same, and prove that 600 × 50 is indeed 30,000.

This lesson is taken from Maria Miller's book Math Mammoth Multiplication 2, and posted at www.HomeschoolMath.net with permission from the author. Copyright © Maria Miller.
Math Mammoth Multiplication 2
A selfteaching worktext for 4th grade that covers multiplying by whole tens and hundreds, multidigit multiplication in columns, order of operations, word problems, scales problems, and money problems.
Download ($5.10). Also available as a printed copy.