Lesson 3.3 Adding and Subtracting Decimals

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Lesson 3.3 Adding and Subtracting Decimals
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Essential Question

• How do I add and subtract decimals?
• Always line up decimals
• Add and subtract like you always do
• Bring decimal straight down in your answers

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Adding

• Examples
• 4.55 11.3

Put the first number on the top of the second
number and line up the decimals.
You can add a zero to help keep everything lined
up
When adding or subtracting you always start from
the right and work left.
4.55 11.3
0
15.85
4
6.44 16
When there is not a decimal put one behind
the number.
Line up the decimal
Add a zero to line up everything
6.44 16.
0
22.44
5
-8.33 -10.972

• -8.33
• -10.972

Line up the decimal
Add a zero to line up everything
0

Same signs Add keep the sign
-19.302
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Subtracting

• Examples
• 5.34 -2.08

Put the first number on the top of the second
number and line up the decimals.
Different signs Take difference
5.34 -2.08
3.26
7
-38.59 (-7.81)

• -38.59
• 7.81

Line up the decimal

-30.78
Subtract to add when negatives are
used. Different signs take difference
8
28 15.911
When there is not a decimal put one behind
the number.
Add zeros to line up everything, then subtract
7
9
9
28. -15.911
000
.
8
0
2
1
9
9
Magic Square
0.5 7
4.625
8.75
The sum of each row, column, and diagonal must be
the same.
6.375 0.5 7
5.25 4.625 4
2.25 8.75 2.875
Sums to 13.875
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Estimation

• You can always estimate to know if your answer is
  close.
• 6.7 8.2
• Ask yourself, Is 6.7 closer to 7 or 6?
• Ask yourself, Is 8.2 closer to 8 or to 9?
• Your estimate is 7 8 ________
• Actual answer?? _______________

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8
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Estimate and solve the following(E Estimate,
A Actual)

• 8.9 3. 4 E______ A _______
• 7.4 2.1 E ______ A _______
• 6.7 4.3 E ______ A _______
• 2.9 1.8 E ______ A _______

Watch the signs!!
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• Ms. Kim is making 3 different kinds of cookies
  using chocolate chips. The recipes ask for
• 2.25 kg, 1.75 kg, and 4.55 kg of chocolate chips.
  Ms. Kim has 11 kg of chocolate chips total.
• Estimate to find out how much Ms. Kim needs.
  Then find the actual amount.
• Does she have enough to make the cookies?
• If your answer is NO, how much more does Ms.
  Kim need to make the cookies? If your answer is
  YES, how much does she have leftover?