Åk 6–9

4.1 Increase
Percent is often used to describe how much something increases or decreases. If can be for example about prices, taxes or salaries. You also use percent when you talk about how much new clothes will shrink when you wash them or when you talk about how a population in a country has increased.

 The DVD player in the picture cost 2,000 crowns.  After New Year’s the store will raise its prices by 20 %.  How much will the DVD players cost then? Here are three different suggestions of how you can solve this problem: Foto: Fredrik Enander 1. First you can calculate the answer in two steps: How much is 20 % of 2,000 kr? Convert 20 % to a fraction. It becomes a fifth, 1/5. One fifth of 2,000 is 2,000/5 = 400 kr. If the price is raised by 20 % then it is 400 kr more. The new price becomes 2,000 kr + 400 kr = 2,400 kr. 2. A second method, is to convert the percentage into decimal form and then multiply: 20 % of 2,000 kr: 0.20 · 2,000 kr = 400 kr The new price is then 2,000 kr + 400 kr = 2,400 kr 3. Or you can think this way: The new price will be 100 % + 20 %. This is the same as 120 %. If you convert 120 % to decimal form, it becomes 1.20. The new price can then be calculated like this: 1.20 · 2,000 kr = 2,400 kr. Directly multiply the number 1.2 like we did in the example above is called calculating with the percent of change which we will look more into in chapter 4.3. Percent Increase If we calculate a percentual increase, the same rules apply as in section 3.1. There we said: Ex: A DVD player costs 2,000 kr. Then the price of the player increases by 250 kr.  By how many percent has the DVD player’s price increased? The part in this case is the change in price, and the whole is the original price (the old price). We then get: Rate of change: 250 kr/2,000 kr = 0.125 = 12.5 % Answer: The DVD player’s price has risen by 12.5 %.