1. DO NOW Find the sum or difference. -2 + 6 2) -3 + (-4) 3) 7 + (-5)
) (-4) ) (-5)
4) -2 – ) 76 – (-42) 6) -21 – 15

2. Learn to add and subtract decimals.

3. Next, they MUST HAVE the same decimal length.
In order to add and subtract decimals, you must first line up the decimals
Next, they MUST HAVE the same decimal length.
To increase the number of decimal places, you can add zeros after the decimal.
If a number does not have a decimal,
put a decimal at the end of the number.
Add zeros to hold the place value
Ex:

4. 66.8 + 4.0 70.8 Example: Find the sum. 66.8 + 4
Use zero as a placeholder so that
both numbers have the same number
of digits after their decimal points.
70.8
Add each column just as you would
add integers.
Line up the decimal points.

5. Example A
Find the sum.
6.44
Line up the decimal points.
Use zeros as placeholders.
22.44
Add.

6. –8.33 + (–10.972) Example A Find the sum. -8.33 – 10.972
8.330
Line up the decimal points.
Use zeros as placeholders.
19.302
Add.
Use the sign of the two
numbers.
–19.302

7. Example A
Find the sum.
(–9.65)
3.6 – 9.65
9.65
- 3.60
Line up the decimal points.
-6.05
Subtract. Answer is negative

8. When adding and subtracting with decimals, it is helpful
to estimate your answer so that you know if you have
done the operation correctly and which sign your answer will have.

9. Example B
Find the difference.
5.34 – 2.08
5.34
Line up the decimal points.
–2.08
3.26
Subtract.

10. Use zeros as placeholders. –15.911 Line up the decimal points.
Example B
Find the difference.
28.000
Use zeros as placeholders.
–15.911
Line up the decimal points.
Subtract.

11. Find the difference.
-32.6 – (-15.86)
32.60
Line up the decimal points.
–15.86
-16.74
Subtract.

12. Extra Example
During one month in the United States,
million commuter trips were taken on buses,
and million commuter trips were taken
on light rail. What was the total number of trips
taken on buses and light rail? Estimate to check
whether your answer is reasonable.
Use zero as a place holder.
Line up the decimal points.
Add.
The total number of trips was million.

13. Extra Example
In 1999, million bushels of corn were
grown in the United States. In 2000, the harvest
yielded million bushels. What was the
total production for those two years? Estimate
to check whether your answer is reasonable.
Use zero as a place holder.
Line up the decimal points.
Add.
The total number of bushels was million.

14. Lesson Quiz
Find the sum.
2. – (–19.2) 1.
(-14.27)
-7.29
–27.66
Find the difference.
-6.72 – (-4.16)
– 3.8 3.
-27.8
-2.56

15. Lesson Quiz
5. Renee finished her first lap in the 200 m freestyle
event in seconds. She completed the second lap in seconds. By how much did her time increase in the second lap? Estimate to check whether your answer is reasonable.
1.41 seconds

16. 6. Identify the sum and then estimate to check whether the answer is reasonable.
A ; estimate: = 20
B ; estimate: = 20
C ; estimate: = 19
D ; estimate: = 19

17. 7. Identify the sum and then estimate to check whether the answer is reasonable.
– (–18.1)
A. –8.78; estimate: –9 + (–18) = –9
B. –27.42; estimate: –9 + (–18) = –9
C. –27.42; estimate: –9 + (–18) = –27
D. –8.78; estimate: –9 + (–18) = –27

18. 8. Identify the difference and then estimate to check whether the answer is reasonable.
7.63 – 3.25
A. 4; estimate: 7 – 3 = 4
B. 4.22; estimate: 7 – 3 = 4
C. 4.32; estimate: 8 – 3 = 5
D. 4.38; estimate: 8 – 3 = 5

19. 9. Identify the difference and then estimate to check whether the answer is reasonable.
35 – 2.7
A. 32; estimate: 35 – 3 = 32
B. 32.3; estimate: 35 – 3 = 32
C. 33; estimate: 35 – 2 = 33
D. 33.3; estimate: 35 – 2 = 33

20. 10. Margaret took an exam that has two parts
10. Margaret took an exam that has two parts. She completed the first part in minutes and the second part in minutes. By how much did her time increase in the second part? Estimate to check whether the answer is reasonable.
A. 16 min; estimate: 33 – 17 = 16
B min; estimate: 32 – 17 = 15
C min; estimate: 32 – 18 = 14
D. 10 min; estimate: 30 – 20 = 10