A modern person is so accustomed to using a calculator that, remaining without this device, sometimes falls into a stupor when it is required to perform even simple computational operations. In our review, 7 simple and effective ways to calculate arithmetic that may not have been covered by school teachers.

## 1. Multiplication by five

To easily and easily multiply by 5, you can first divide the number by two, and then multiply by 10. However, it is easier for someone to count "/>

## 2. Multiplication by four

Multiplying by 4, you can not remember the table, but double multiply by two. Many people, by the way, do it intuitively.

## 3. Checking for multiples of three

Finding out whether a number is divisible by 3 is as easy as shelling pears: the sum of its digits must be divisible by 3. That's it - easy and simple.

## 4. Multiplying three-digit numbers by 1001

It's easy to multiply three-digit numbers by 1001. After multiplying, you get a number in which the first factor is repeated two times: 456 x 1001 = 456456.

## 5. Just multiply by 11

Multiplying a two-digit number by 11 is easier than it sounds. The result of any multiplication is the displacement of two digits of the multiplied and placing between them in the cent of the sum of the displaced digits. For example: 11x27 = 297. If the sum of the extreme digits is greater than 10, then 1 is transferred to the left number, as when calculating in a column: 47 x 11 = 517.

## 6. Multiplication by 9

Multiplying by 9 is easy. It is enough to multiply by 10, and then subtract the multiplied number from the result. For example: 23 x 9 = (23 x 10) - 23 = 207.

## 7. Squaring

It is difficult to call this method useful, as it is rather "interesting". Squaring any number is the sum of consecutive odd numbers, the number of which is equal to the number being squared. For example: 2 x 2 = 1 + 3 = 4; 3 x 3 = 1 + 3 + 5 = 9; 5 x 5 = 1 + 3 + 5 + 7 + 9 = 25.

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