How to learn to quickly count?
It would seem, who in our time of modern technology needs such a skill? After all, you can always take a calculator, which is available in any mobile phone, and carry out the necessary calculations. However, sometimes the desired device may not be nearby. Here you will have the useful ability to quickly count in your mind.
There is a certain set of arithmetic algorithms, each of which, in one way or another, makes it easier to count. Start learning fast counting in the mind is necessary with the memorization of these algorithms.
Subtraction 7, 8, 9
If you need to subtract 9, 8, or 7 from any number, subtract 10 from it, and then add 1, 2, or 3, respectively. For example: 52-9 = 52-10 + 1 = 43.
Multiplication by 9
To multiply any number by 9, first multiply it by 10, that is, simply add a zero at the end, and then subtract the first factor from the result (that is, the number that we multiply).
For example, 72 * 10 (= 720) - 72 = 648.
Multiplication and division by 2
This would seem to be a very simple action, but when it comes to large numbers, everything becomes difficult. It is especially problematic to multiply non-circular numbers.150 multiplied by two is very simple, but 146 is much harder. In this case, round off 146 to 150, multiply, and then subtract from the result the difference between the rounded and unrounded numbers multiplied by two. I.e:
- 150*2 (=300) - 4*2=292.
Do the same when dividing:
Division and multiplication by 4 and 8
The division and multiplication by 4 and 8 are, respectively, doubled and quadruple division and multiplication by two, so, it is better to perform these actions sequentially. That is: 16 * 4 = 16 * 2 (= 32) * 2 = 64.
Multiplication by 5 and 25
To quickly multiply by five, multiply the number by 10, and then divide it in half. For example: 25 * 5 = 25 * 10 (= 250) / 2 = 125.
To multiply by 25, divide the number by four, and then multiply it by 100. For example: 16 * 25 = 16/4 (= 4) * 100 = 400.
Multiplication by single digits
To quickly multiply any number by a single digit, it is convenient to perform sequential multiplication. For example: 84 * 8 = 80 * 8 (640) + 4 * 8 (32) = 672
or 153 * 6 = 100 * 6 (600) + 50 * 6 (300) + 3 * 6 (18) = 918.
Of course, at first glance, these algorithms may seem to you too confused, in addition, you may even think that some actions can be performed much faster without using them. However, this is only as long as your actions on the algorithm will not be brought to automatism.
More lessons can be found here.