1. Lesson 3.3 Adding and Subtracting Decimals

2. 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

3. You can add a zero to help keep everything lined up
Adding
Examples:
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.
15.85

4. Add a zero to line up everything
When there is
not a decimal
put one
behind the
number.
Line up the decimal
Add a zero to line up everything
22.44

5. Add a zero to line up everything
Line up
the decimal
Add a zero to line up everything +
Same signs
Add – keep the sign

6. Subtracting Examples: 5.34 -2.08 5.34 -2.08 3.26
Put the first number on the top of the second number and line up the decimals.
Different signs
Take difference
3.26

7. -38.59 – (-7.81) -38.59 + 7.81 -30.78 + Line up the decimal
Subtract to add when negatives are used.
Different signs take difference

8. Add zeros to line up everything, then subtract
28 –
When there is
not a decimal
put one
behind the
number.
Add zeros to line up everything, then subtract 7 9 9
000 . 1 2 8 9

9. The sum of each row, column, and diagonal must be the same.
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

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

11. Estimate and solve the following… (E = Estimate, A = Actual)
E=______ A = _______ 7.4 – 2.1 E= ______ A = _______ 6.7 – 4.3 E= ______ A = _______ E= ______ A = _______
Watch the signs!!

12. Ms. Kim is making 3 different kinds of cookies using chocolate chips
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?