Pre Algebra Decimal Numbers
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What's the Point?
Decimal Numbers

 
 When we write "one dollar" we sometimes write:
 

 $1.00

 That is because our money is divided up 
 into pieces of dollars called cents. 
 
 The number on the right of the period 
 (in math we call it a decimal point) 
 are for those pieces of a dollar. 
 
 To write a quarter (also known as 25 cents), we can write:
 

$0.25

 
 Two quarters (sometimes known as 50 cents but never 50 cent) 
 looks like this:
 

$0.50

 
 The "S" with a line through it is just a code. 
 This code says we're talking about dollars or pieces of dollars. 
 It's not really part of the number.
 
 We could use addition and add two quarters 
 to get 50 cents like this:
 

 
 If we start with two quarters (fifty cents) 
 and add another two quarters (fifty cents),
 it looks like this:

 

 
 No big surprise here. 
 Most people know that 4 quarters is the same as a dollar. 
 The big thing is to see how these numbers add together.
 
 WHEN YOU ADD THESE NUMBERS,
  THE DECIMAL POINTS MUST LINE UP.
 
 Example:
 
 Add 38 cents plus 24 cents:
 

 
 We can do subtraction with this stuff too!
 
 Example:
 
 81 cents minus 30 cents:
 

 
 The word cent comes from some foreign language. 
 It means 100. 
 There are 100 cents in a dollar. 
 This is like a pizza with 100 slices. 
 Each cent is one slice. 
 
 In our "slice of pizza" way of writing numbers, 
 we could write one cent like this:
 

 
 Using our new decimal point way, we can write one cent like this:
 

$0.01

 
 But hey! A penny is a penny no matter how you write it so:
 

 
 There are 4 quarters in one dollar. 
 Using the "pizza slice" fraction method, 
 we can say that there are 4 quarter sized slices in one dollar. 
 So for ONE quarter we can write:
 

 
 Using our new decimal point method 
 we can write one quarter (25 cents) as:
 

  $0.25

 And a quarter is a quarter so:
 

 

 Example:

 
 There are 10 dimes in a dollar (like a pizza with 10 slices) 
 each dime is also worth 10 cents. 
 If we wanted to add two dimes together we could write:
 

 

 
 And we could also write:
 

 

   copyright 2005 Bruce Kirkpatrick

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