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Terrabyte – The Digital Hub

Department of Computer Science

Speed Maths

Maths. It is something that none of us can avoid, even  after we leave school. Whether it is working out how much money we have to spend each month once the bills have been paid, or working out if we have enough money in our pocket for a couple of magazines at the local shop, being able to do mental arithmetic is really a useful skill.

Sadly, lots of people are put off from improving  their maths skills. This might be because sums look daunting on paper. At school methods of learning sums and doing mental arithmetic can be a bit  threatening, boring or just not interactive enough.

Here we tell you something about “Speed Maths”, which will help improve your maths and the speed at which you can do sums mentally. You can become a maths wizard  in no time at all!

All you need is the ability to concentrate and the determination that you want to improve.

The great thing is, your current level doesn't matter. The great thing about Speed Maths is that you notice a general improvement in both the speed and accuracy with which you can perform various mental sums.

Let us begin with an example

Consider 989 * 25

1.      Halve 989 (Divide by 2; Do not forget to take Decimal points. The result of halving is 494.5

2.      Halve 494.5. The result is 247.25

3.      Multiply the result 247.25 by 100. The result will 24725. (Move the Decimal point two places to the right). The result obtained is 24725.

4.      Therefore 989 * 25 = 24725.

Interesting? Shall we proceed further?

Let us try some percentage problems. For many, percentages are confusing. 

First lets look at how to read it: 50% is read as "Fifty per cent".

 The 'per cent' bit means 'parts of a hundred' so think of 50% as being fifty parts of a hundred - in other words, it is exactly half of something. So 50% of 20 is half of 20, which is 10. 

So how do we do simple percentage sums, then? Let's look at one:

Calculate 40% of 50. 

It's always a good idea to turn maths which is abstract into a real life example. So we can think of this as saying "what is forty percent of 50 oranges"?

Now, imagine the old game where you say "one for me, one for you" and dividing those oranges up. But this time, for every 4 oranges for me, we feel generous and give you 6. This means that for every ten oranges we have, you get 6 and I get 4. To get from ten oranges to fifty oranges, you multiply by five, so I end up with 4 x 5 oranges which is of course 20 oranges. So that's the answer.

If you read and followed the above paragraph, your key question will probably be "but how do I get from knowing that for every 10 oranges, I keep 4?" 

Great question! And understanding that is the heart to doing percentage sums. Remember that the '%' sign means 'per hundred' and so 40% means '40 from every 100'. And our specific sum asks us to calculate this from 50 oranges. So what we need to do is work out the relation between 100 and 50. If we take 40 oranges for every 100, then how many oranges do we take from 50?

All we need to do is divide 50 by 100, to get 1/2. We then multiply this by 40 to find how many I get from 50, and find the answer is 20.

Do you Understand how this works? 

This is really important to get your head round, but can be hard. Read through the above until you understand what a percentage means and what the method to solve the question is and why it works. Understanding percentages and what they represent is the key to being able to solve percentage sums - just learning the method is not the best way of solving them though it gives you the answer. 

Here's one final representation of this sum for you:

- If I have 100 oranges, and I take 40 of 100, how many do I have? Answer: 40 oranges! 
- If I have 50 oranges, and I take 40 of 100, how many do I have? Answer: 20 oranges! 

Final recap: to calculate a percentage like this, simply divide the number of items we have by 100, and multiply by the percentage. So, 40% of 50 becomes this sum: (50 / 100) x 40. This equals 1/2 x 40 which is 20.

Try some more on your own and let us know if it works.                                                                                                                                                                                                                                                                                                                                                         Ravinder Murthy, M.Sc IT