Numeracy in the mind – it’s not a talent, and not a gift. It is simply knowledge of some mathematical tricks. Now you will have additional knowledge that will help you use your mental abilities a bit more efficiently.
Trick #1 Tip counting.
If you need to leave a 15% tip, there is a simple way to do it. Calculate 10% (divide by 10), and then add the resulting number to its half and get the answer:
15% ?f $25 = (10% ?f 25) + ((10% ?f 25) / 2)
$2.50 + $1.25 = $3.75
Trick #2 Division by 5.
Actually dividing large numbers by 5 is very simple. All you need – just multiply by 2 and move the decimal point:
195 / 5
Step 1: 195 * 2 = 390
Step 2: Move the decimal point: 39.0 or just 39.
2978 / 5
Step 1: 2978 * 2 = 5956
Step 2: 595,6
Trick #3 Subtraction from 1000.
To perform subtraction of 1000 you can use this simple rule: Subtract from 9 all the numbers except the last. And the last digit subtract from 10:
1000
-648
Step 1: subtract 6 from 9 = 3
Step 2: subtract 4 from 9 = 5
Step 3: subtract 8 from 10 = 2
352
Trick #4. Fast squaring, better method:
a^2 = (a-b)(a+b)+b^2
So, e.g:
291 = (291-9)(291+9) + 9^2 = 282*300+81 = 84600+81 = 84681
With a little practice, you can do this in your head.
Trick #5. Calculating 10 x, where x is a non-integer.
The first thing is that x can always be broken down into x = k+y, where k is an integer and y is between 0 and 1. Then, obviously, 10k+y = 10k *10y.
So the question becomes how do you do 10 y where y is between 0 and 1? Well, you can figure it out using some tricks when y = 0.1, 0.2, …, 0.9, and then interpolate between those. For example, say you need to get 100.3. Then 210 = 1024 ~= 103. So 210/10 ~= 103/10 = 100.3. So 100.3 ~= 2 (correct answer is 1.99). Then knowing 100.3 makes figuring out 100.6 and 100.9 easy. Also, 100.5 is easy, because 210 ~= 103, so 25 ~= 103/2 so 25 /10 = 3.2 ~= 100.5.
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