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To express an amount as a percentage, write it as a fraction of the total amount and multiply it by 100. Here's an example!
There are 14 boys and 11 girls in a class.
What percentage of the class are boys?
Total in the class = 25 so the fraction of boys is 14/25.
So the percentage of boys in the class is 56%.
To find a percentage of a quantity, turn the percentage into a decimal (divide by 100) and multiply the quantity by this. Here's an example:
Find 55% of 230km.
The simple way to increase (or decrease) a quantity by a percentage is to find the percentage of the quantity that you want to increase (or decrease) it by and add it on (or take it off).
Increase £56 by 25%.
25% of £56 is £14
56 + 14 = 70 so the answer is £70.
You can actually do it in one step instead of two!
1. Work out what percentage you will have after your increase or decrease.
2. Change it to a decimal (divide by 100).
Have a look at this example:
Decrease 900kg by 35%.
After taking away 35% there will be 65% left.
65% as a decimal is 0.65 so 0.65 x 900 = 585
The answer is 585kg.
Let's say you have £10 in the bank at 10% interest per annum (that means per year!).
So at the end of one year you will get £1 interest giving you a balance of £11.
At the end of the second year you now get 10% of £11 i.e. £1.10 so you are getting interest on your interest (if you see what we mean!). This is called Compound Interest.
So if you want to find out how a sum of money has grown due to interest added over a number of years, here's what you do:
1. Add the interest rate on to 100% to get your new percentage.
2. Change it to a decimal. This is now the number you multiply by at the end of each year.
A man has £50 in his bank account at the start of the year. The interest rate is 5.3% per annum. If he leaves his money there, how much will the balance be at the end of 3 years?
If you add 5.3% on you will have 105.3%.
As a decimal this is 1.053. So we simply multiply the £50 by this number 3 times as there are 3 years. In other words we are multiplying by (1.053)3.
50 x (1.053)3 = 58.37879385
Now remember money has two decimal places so the answer is £58.38.
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