Transcript Chapter 12

Multiply and Divide Decimals

Round to the ones place.
Example: Estimate 15.8 x 5
15.8 → 16
x 5→ x5
80


When multiplying a decimal by a whole
number, place the decimal point in your
answer directly below where it is in the
problem.
Examples –
1.) 2 × 0.53
0.53
× 2
1.06
2.) 12 × 0.7
12
× 0.7
8.4
 When multiplying a decimal by a decimal, move the
decimal points all the way to the right to make it a whole
number. In your answer, move the decimal point the
same number of places to the left.
0.52
x 0.6
52
x6
312.
2 decimal places
+ 1 decimal place
3 decimal places
312. = 0.312 – move decimal point 3 places to the
left.
*If there are no more numbers on the left, you must add
zeros.

Commutative Property of Multiplication
 The order in which the factors are multiplied does not
change the product.
 Ex. –
5x4=4x5

Associative Property of Multiplication
 The way in which the numbers are grouped does not
change the product.
 Ex. –
(7 x 3) x 9 = 7 x (3 x 9)

Identity Property of Multiplication
 The product of any number and 1 equals the number.
 Ex. 12 x 1 = 12

Use compatible numbers to divide.
Remember, the numbers must go in evenly –
no remainders!

Example:
$17.50 ÷ 7 → $ 18 ÷ 7
*Does not go in evenly – pick different
numbers
$21 ÷ 7 = $3
or
14 ÷ 7 = 2

You DO NOT need to use models. The steps are simple.

Write the decimal point in your answer DIRECTLY ABOVE
where it is in the problem.
↓
__.__
3) 4.2

Divide as you would divide a whole number.
_1.4__
3) 4.2
-3 ↓
12
-12
0
1.
2.
3.
Move the decimal point in the divisor to make it a whole
number.
Move the decimal point in the dividend the same number
of places.
Place the decimal point and divide.
_____
o.05) 4.205
_ 84.1
5) 420.5
-40↓
20
-20↓
05
- 05
0
→
______
o05.) 420.5
↑ 2 places ↑
to the right