Percent

• Percent is parts per hundred.
• It is really a ratio to the number 100. 23% is 23/100
• It provides a way to compare things in different quantities.
  • Quiz scores: Quiz 1: 18/20 = 90%. Quiz 2: 23/25 = 92%
• Fraction to decimal to percent to
  • 25% = .25,
  • Drop the % sign and divide by 100
  • .25 = 25%
  • multiply by 100 and add a %
  • Parts per whole
  • 15/30 = .50 = 50%
• Percentage change tells us how much something has changed.
  • Formula: percent change = (new value - old value)/old value x 100
  • Also know as percent increase, decrease
  • A positive percent change means the thing went up.
  • A negative percent change means the thing went down.
  • Example:

    If there were 7,700 students at Edinboro in 2008 
    and 8,200 students in 2010, what is the percentage 
    change and is it an increase or decrease?
    
           PC = (new-old)/old x 100
              = (8200 - 7700) / 7700 x 100 
    	  = 500/77 
    	  = 6.5% 
    
    	  It is a percent increase.
    	

• Percent Markup tells us how much a seller makes.
• markup = (selling price - cost)/cost x 100

  If I pay $4.35 for a box of rocks, which I sell for $6.89.
  What is my markup?
  
  markup = (6.98-4.35)/4.35 x 100 
         = 2.63/4.35 x 100 
         = .60 x 100 
         = 60%
      

• Examples 7, 11, 12, 13, 15, 21, 24, 26, 29, 57, 63

Personal loans and simple interest

• When you borrow money you
• The principal is the amount of money you get.
• The interest rate is a percent of the principal you pay as rent on the money
• The interest is the amount of money you pay as rent.
• The period, or time is the amount of time it takes to pay off the loan.
• i = prt
• Look at problems 9 through 24
• Look at problem 27
• Discount loans
  • These are different from simple interest loans in that the interest is charged in advance.
  • If you receive a loan for $9,500 at 3% for 9 months, discounted, you have to pay $9,500 at the end of the loan.

    i = prt
    i = $9,500 x .03 x 3/4
    i = $213.75
    
    So the interest was 213.75, 
    The amount loaned to the person was 9,500-213.75
    9,286.25
    
    And the interest rate was really
    i = prt
    r = i/pt
      = 213.75/(9,286.25*3/4)
      = .0307
      = 3.07%
    	  

• Examples: 9, 11, 12, 13, 19, 21, 23, 26, 27, 28