Percentage-Increases-And-Decreases Wjec-Eduqas Foundation

Percentage increases & decreases

How do I increase by a percentage?

• A percentage increase makes an amount bigger by adding that percentage on to itself

• Without a calculator, use the the basic percentages methods to find the percentage you are increasing by

  • Then add this on to the original amount 

  • To increase 30 by 10%

    • 10% of 30 is 3

    • 30 + 3 = 33

    • This is equivalent to finding 110% of 30

• With a calculator it is more efficient to use multipliers

  • A multiplier is the decimal equivalent of a percentage

    • A percentage can be converted to a decimal by dividing by 100

  • When increasing by a percentage, we are finding a percentage greater than 100%

  • To increase 80 by 15%

    • We are finding 115% of 80, so the multiplier is 1.15

    • 1.15 × 80 = 92

How do I decrease by a percentage?

• A percentage decrease makes an amount smaller by subtracting that percentage from itself

• Without a calculator, use the methods outlined in Basic Percentages to find the percentage you are decreasing by

  • Then subtract this from the original amount 

  • To decrease 30 by 10%

    • 10% of 30 is 3

    • 30 – 3 = 27

    • This is equivalent to finding 90% of 30

      • Because 100% – 10% = 90%

• With a calculator it is more efficient to use multipliers

  • When decreasing by a percentage, we are finding a percentage smaller than 100%

  • To decrease 80 by 15%

    • We are finding 85% of 80, so the multiplier is 0.85

      • Because 100% – 15% = 85%

    • 0.85 × 80 = 68