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Chapter 3 · Decimals and percents · page 18 of 68

Percent change and discounts

levelPre-algebra
standard7.RP.A.3
practiceDecimals and percents

Percent change compares the change to where you started. The original number is always the bottom of the fraction. A discount is a percent of the original price, taken off.

How to use it

  1. Find the change: new minus original, ignoring the sign.
  2. Divide by the original.
  3. Multiply by 100.
  4. For a discount: find the percent of the price, then subtract it. Or multiply by what is left: 25% off means paying 75%.

Example

40→50:  1040=0.25=25% increase40 \to 50: \; \frac{10}{40} = 0.25 = 25\% \text{ increase}
$60 at 25% off:  60−15=45\$60 \text{ at } 25\% \text{ off}: \; 60 - 15 = 45
0.75×60=450.75 \times 60 = 45

Where it goes wrong

  • Dividing by the new number instead of the original.
  • Reporting the discount amount as the sale price.
  • Thinking 25% up then 25% down gets you back. 40 up 25% is 50, and 50 down 25% is 37.5.

Related

Percent of a numberPercent means per hundred
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