Multiplying Whole Numbers and Decimals
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Transcript of Multiplying Whole Numbers and Decimals
Multiplying Whole Numbers and Decimals
Lesson 2-2
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
Strategy #1405 • 59 = x
405x 59
or 405x 50
405x 9
+
455 x 9 = 45
Put the 5 in the answer and carry the 4 (it’s really 40) to the next place value (tens)
4(9 x 0) + 4 = 4
Write down the 4.
36(9 x 4) = 36
Since there are no other numbers to multiply, write down the 36.
Zero is a place holder. Put a zero in your answer to hold
its place.
0255 x 5 = 25
Put the 5 in the answer and carry the 2 (it’s really 20) to the next place value (tens)
(5 x 0) + 2 = 2
Write down the 2.
02(5 x 4) = 20
Since there are no other numbers to multiply, write down the 20.
2
Strategy #1405 • 59 = x
405x 59
or 405x 50
405x 9
+
45436 02502 2
23,895The final answer is:
Strategy #2The Lattice
405x 59
There are 3 digits in this number
There are 2 digits in this number
We will start by making a 3 x 2 window…
Strategy #2The Lattice
405x 59
3 digits
2 digits
Strategy #2The Lattice
405x 59
4 0 5
5
9
x
Put diagonal lines in each of the windows.
Strategy #2The Lattice
405x 59
4 0 5
5
9
x
Multiply one number from the top with one from the side. Put your answer in the box where they meet.
4 x 5 = 20
2 0
Strategy #2The Lattice
405x 59
4 0 5
5
9
x
5 x 0 = 0
2 00 0
Strategy #2The Lattice
405x 59
4 0 5
5
9
x
5 x 5 = 25
2 00 0 2 5
Strategy #2The Lattice
405x 59
4 0 5
5
9
x
4 x 9 = 36
2 00 0 2 5
3 6
Strategy #2The Lattice
405x 59
4 0 5
5
9
x
0 x 9 = 0
2 00 0 2 5
3 6 0 0
Strategy #2The Lattice
405x 59
4 0 5
5
9
x
5 x 9 = 45
2 00 0 2 5
3 6 0 0 4 5
Strategy #2The Lattice
405x 59
4 0 5
5
9
x2 0
0 0 2 53 6 0 0 4 5
Now we will add numbers diagonally from right to left…
Strategy #2The Lattice
405x 59
4 0 5
5
9
x2 0
0 0 2 53 6 0 0 4 5
598322
3 8 9 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.
If my problem has decimals…
4.05x 5.9
23,895
4.05x 5.9
405x 59
23.895
.405x .059
.405x .059
.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.
Let’s try one more…
Strategy #2The Lattice
395x 84
There are 3 digits in this number
There are 2 digits in this number
We will start by making a 3 x 2 window…
Strategy #2The Lattice
3 digits
2 digits
395x 84
Strategy #2The Lattice
3 9 5
8
4
x
Put diagonal lines in each of the windows.
395x 84
Strategy #2The Lattice
3 9 5
8
4
x
Multiply one number from the top with one from the side. Put your answer in the box where they meet.
8 x 3 = 24
2 4 395x 84
Strategy #2The Lattice
3 9 5
8
4
x
8 x 9 = 72
2 47 2 395
x 84
Strategy #2The Lattice
3 9 5
8
4
x
8 x 5 = 40
2 47 2 4 0 395
x 84
Strategy #2The Lattice
3 9 5
8
4
x
4 x 3 = 12
2 47 2 4 0
1 2
395x 84
Strategy #2The Lattice
3 9 5
8
4
x
4 x 9 = 36
2 47 2 4 0
1 2 3 6
395x 84
Strategy #2The Lattice
3 9 5
8
4
x
4 x 5 = 20
2 47 2 4 0
1 2 3 6 2 0
395x 84
Strategy #2The Lattice
3 9 5
8
4
x2 4
7 2 4 01 2 3 6 2 0
Now we will add numbers diagonally from right to left…
395x 84
Strategy #2The Lattice
3 9 5
8
4
x2 4
7 2 4 01 2 3 6 2 0
081333
3 8 0
395x 84
111
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.
If my problem has decimals…
39.5x 8.4
33180
39.5 x 8.4331.8
0
.395x .084
.395x .084
.033180
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.
395x 84
Let’s Practice…
78.6 x 9837
9
8
8 6
3
6 3 7 2 5 45 6 6 4 4 82 1 2 4 1 8
83
1
6
1
2
2
7
17
77263.8
There is one digit after a decimal point.
772638
.392 x .0083
0
0
9 2
8
0 0 0 0 0 00 0 0 0 0 02 4 7 2 1 6
631
1
3
0
0
.003136
There are 6 digits after a decimal point.
003136
9358 x .3
2807.4
115.6 x .004
.4624
.0018 x 7
.0216