Lesson 11-1

Example 1 Multiply Decimals
Find 15.3  4.

METHOD 1  Use estimation.

Round 15.3 to 15.
15.3  4 → 14  4 or 56

 2 1
15.3		
   4
61.2	Since the estimate is 56, place the decimal point after the 1.

METHOD 2  Count decimal places.

one decimal place

 2 1
15.3	
   4

Count one decimal place from the right.

61.2	

Example 2 Multiply Decimals
Find 7  0.72.

METHOD 1  Use estimation.

Round 0.72 to 1.
7  0.72 → 7  1 or 7

5   1
0.72		
   7
5.04	Since the estimate is 7, place the decimal point after the 5.

METHOD 2  Count decimal places.

two decimal places

5   1    
0.72	
   7

Count two decimal places from the right.

5.04	

Example 3 Annex Zeros in the Product
Find 6  0.013.

three decimal places

       1 
 0.013	
      6
 0.078	

Annex a zero on the left of 78 to make three decimal places.

Example 4 Annex Zeros in the Product
ALGEBRA   Evaluate 2m if m = 0.0043.

2m = 2  0.0043	Replace m with 0.0043.

four decimal places

0.0043			
       2
0.0086			Annex two zeros to make four decimal places.	

Example 5 Scientific Notation
Write 8.2 × 105 in standard form.

METHOD 1  Use order of operations.

Evaluate 105 first. Then multiply.
8.2 × 105 = 8.2 × 100,000
	= 820,000

METHOD 2  Use mental math.

Move the decimal point to the right the same number of places as the exponent of the power of 10, or 5 places.
8.2 × 105 = 8.20000 or 820,000