1. Adding, Subtracting, and Multiplying Decimals 3-1/3-2
Notes 6
Adding, Subtracting, and Multiplying Decimals
3-1/3-2

2. Adding and Subtracting Decimals
66.8
+ 4.0
Use zero as a placeholder so that
both numbers have the same number
of digits after their decimal points.
70.8
Add (or subtract) each column just as
you would add (subtract) integers.
Line up the decimal points.
The normal average summertime temperature in the Midwest is 70.8°F.

3. Additional Example 1A: Adding Decimals
Add. Estimate to check whether each answer is
is reasonable.
4.55
Line up the decimal points.
Use zero as a placeholder.
15.85
Add.
Estimate
= 16
15.85 is a reasonable answer.

4. When adding numbers with the same sign, find the sum of their absolute values. Then use the sign of the numbers.
Remember!

5. Additional Example 1C: Adding Decimals
Add. Estimate to check whether each answer
is reasonable.
– (–10.972)
– (–10.972)
Think:
8.330
Line up the decimal points.
Use zero as a placeholder.
19.302
Add.
– (–10.972) = –19.302
Use the sign of the two
numbers.
Estimate
–8 + (–11) = –19
– is a reasonable
answer.

6. Additional Example 2: Subtracting Decimals
5.34
Line up the decimal points.
–2.08
3.26
Subtract.
B. 28 –
28.000
Use zeros as placeholders.
–15.911
Line up the decimal points.
12.089
Subtract.

7. Multiplying Decimals 7  8 56 0.7  0.8 0. 56 1 decimal place
To multiply decimals, multiply as you would with integers. To place the decimal point in the product, count the number of decimal places in each factor. The product should have the same number of decimal places in the factors. 7
 8 56
0.7
 0.8
1 decimal place
1 decimal place + 0. 56
2 decimal places
Same digits

8. Additional Example 1: Multiplying Integers by Decimals
0 decimal places 7
 0.1
1 decimal place
0.7
0 + 1 = 1 decimal place
B. –3 · 0.03
0 decimal places –3
 0.03
2 decimal places
– 0.09
0 + 2 = 2 decimal places. Use zero
as a place holder.