420  1080  1380  36000  
10%  
50%  
25%  
33⅓%  
75%  
90% 
Topics: Starter  Arithmetic  Mental Methods  Percentages
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Previous Day  This starter is for 16 October  Next Day
%  This is the percent symbol.
Percent means 'out of 100'.
As 50 is half of 100, then 50% means half. To find 50% of a quantity you need to halve (or divide by two). So 50% of 6 is 3.
As 10 is one tenth of 100, then 10% means 'one tenth of'. To find 10% of a quantity you need to divide it by ten. So 10% of 800 is 80.
As 25 is one quarter of 100, then 25% means 'one quarter of'. To find 25% of a quantity you need to divide it by four. So 25% of 20 is 5.
Another way of finding 25% of a quantity is first finding 50% then dividing the result by 2.
As 33⅓ is one third of 100, then 33⅓% means 'one third of'. To find 33⅓% of a quantity you need to divide it by three. So 33⅓% of 30 is 10.
As 1 is one hundredth of 100, then 1% means 'one hundredth of'. To find 1% of a quantity you need to divide it by 100. So 1% of 800 is 8.
Other percentages can be found by combining some of the techniques mentioned above. Here are some examples:
If you need to use a calculator to check your working. See Calculator Workout skill 3.
Did you know that if you are struggling to mentally work out 24% of 50 you can switch the numbers round and work out 50% of 24 instead. Finding 50% is very easy isn’t it? You will get the same answer.
Finding a percentage of a quantity is an example of a commutative calculation. Not all operations are commutative. Subtraction certainly isn’t as 10 minus one is not the same as one minus ten.
You can use this trick to improve your ability to do this type of calculation quickly if you find the switch makes it easier.
Practise with 12% of 50, 4% of 25 and 75% of 10.
420  1080  1380  36000  
10%  42  108  138  3600 
50%  210  540  690  18000 
25%  105  270  345  9000 
33⅓%  140  360  460  12000 
75%  315  810  1035  27000 
90%  378  972  1242  32400 
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