Multiplying Whole Numbers and Decimals

1
Multiplying Whole Numbers and Decimals

• Unit 3 Lesson 8 and 9
• Powerpoint taken from
• http//www.mrsoregan.net/PwrPnt_Notes.htm

2
Things you need to know

• Do not line up the decimal points!
• Pretend all decimals are whole numbers until the
  very end
• Only look at the decimals when trying to figure
  out where to put the decimal point in your answer

3
Strategy 1
405 59 x
405 x 59
405 x 50
405 x 9

4
5
4
3
6
0
2
5
0
2
2
Zero is a place holder. Put a zero in your
answer to hold its place.
(5 x 0) 2 2 Write down the 2.
(5 x 4) 20 Since there are no other numbers to
multiply, write down the 20.
(9 x 0) 4 4 Write down the 4.
5 x 9 45 Put the 5 in the answer and carry the
4 (its really 40) to the next place value (tens)
5 x 5 25 Put the 5 in the answer and carry the
2 (its really 20) to the next place value (tens)
(9 x 4) 36 Since there are no other numbers to
multiply, write down the 36.
4
Strategy 1
405 59 x
405 x 59
405 x 50
405 x 9

4
5
4
3
6
0
2
5
0
2
2
23,895
The final answer is
5
What if I have a decimal point in my problem?

• Multiply your numbers using one of the steps just
  shown.
• When done, count all the numbers after a decimal
  point in your problem.
• Make sure your answer has exactly that number of
  digits in the decimal spaces.

6
If my problem has decimals
4.05 x 5.9
4.05 x 5.9
405 x 59
.405 x .059
.405 x .059
23,895
23.895
.023895
I had to add a zero because there are six total
decimals spots in my problem, but I only had 5
digits in my answer. Notice that the zero came
between the decimal point and my old answer.
7
Lets try one more
8
What if I have a decimal point in my problem?

• Multiply your numbers using one of the steps just
  shown.
• When done, count all the numbers after a decimal
  point in your problem.
• Make sure your answer has exactly that number of
  digits in the decimal spaces.

9
If my problem has decimals
39.5 x 8.4
39.5 x 8.4
.395 x .084
.395 x .084
395 x 84
.033180
331.80
33180
I had to add a zero because there are six total
decimals spots in my problem, but I only had 5
digits in my answer. Notice that the zero came
between the decimal point and my old answer.
10
Lets Practice
11
78.6 x 983
78.6 X 983 2358
62880 707400 772,638
77263.8
772638
12
.392 x .008
1
There are 6 digits after a decimal point.
.392 X .008 .003136
.003136
003136
13
9358 x .3
2807.4
14
115.6 x .004
.4624
15
.0018 x 7
.0216