11.1 Using mental methods

Some percentages are easy to find because they are simple fractions like. For example, 50% are easy to find because its just 50/100 and its easily dividable.  Some percentages aren't very easy to find because they aren't simple fractions. For example, 21% is not very easy to find because its not a simple fraction(21/100). There are examples of these on the first page of this unit.

You can use the easy ones to work out more complicated percentages.

You can often do this quite easily. You do not always need a calculator.

Have you ever feel so upset because your favorite stuff are on sale and you don't know how to count those discounts without calculators? No worries!

In this unit I will introduce you a method that can make your life a lot easier. It's called a mental method. This method will help you find the value of something that has complicated percentage.

Hint: 

If you know 10%, you can find any multiple of 10%.

Worked example 11.1:

There are 4600 people in a stadium. 58% are males. How many is that?

100% = 4600

58%   = 50% + 10% - 2% [these are all easy percentage to find]

50%   = 2300 [50% = 1/2]

10%   = 460 [1/10 is easy. Just divide by 10]

1%     = 46 [divide 10% by 10 to find 1%]

58%   = 2300 + 460 - (2 x 46) = 2668 [Do this sum on your head or on paper to practice                                                             your mental methods skill]

*You could have found 50% + 5% + 3%. Is that easier? Show your working with these steps and leave a comment below telling me which one do you think is easier! :)

 Now that I have explained the whole thing about this sub-chapter, I think we should try to answer some questions with this method, shall we? :D

Question 1:

Use the fact above to find:
a. 52% of $78 = ?
answer:
52% of $78 is twice the amount of 26% of 78.
so, we can just count it like this: 
52% of $78 = 2(26% of 78)
                 = 2 x 20.28
                 = $40. 52
b. 13% of 78 kg = ?
answer: 
13% of 78a kg is half the amount of 26% of 78.
so, we can just count it like this: 
13% of 78 kg = (26% of 78) / 2
                 = 20.28 / 2
                 = 10.14 kg
Question 2:
150% of 62 = ?
answer:
150%    = 100% + 50%
           = 62 + (62 / 2)
           = 93
Question 3:
0.5% of 7000 = ?
answer:
0.5%    = 1% / 2
           = (1% of 7000) / 2
           = 70 / 2
           = 35

Here is a game to help your understanding about percentages:
http://www.bgfl.org/bgfl/custom/resources_ftp/client_ftp/ks2/maths/percentages/index.htm
Hope it'll help :)