2.2 + – a percentage

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Increasing by a percentage

Percentages can be added, whether they are in fraction, decimal or percentage form – however, the key is that what is being added must be in the same form. For example:

  • In fraction form:
  • Alternatively, 200\%+60\%=260\%
  • Alternatively, 2.00+0.6=2.6

They can be added because they have the same common denominator of 100.

To increase a number by a certain percentage, find (100+that increase) percent of the original number.

For example, to increase a 2900 by 20%, we find 120% of 2900. Knowing that 120\%=1.2, we multiply 1.2*2900, i.e.:

Note the decimal place hasn’t been inserted yet! As the question had a total of 1dp, we move the decimal place one to the left. So, the answer is 3480, the number 2900 increased by 20%.

Decreasing a percentage

Percentages can be subtracted in the same way they can be added, for example:

  • In fraction form:
  • 300\%-90\%=210\%
  • 0.300-0.9=2.1

To decrease a number by a certain percentage, find (100-that decrease) percent of the original number.

For example, to decrease 2900 by 30%, find 70% of 2900. As 70% is 0.7, we multiply 0.7*2900, i.e.:

Note the decimal place hasn’t been inserted yet! As the question had a total of 1dp, we move the decimal place one to the left. So, the answer is 2,030, the number 2900 decreased by 30%.

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