Like Nodas said, here.

I have been writing about fast methods her on this forum for the past 8 years.

It’s a kind of abacus where they put the numbers on their fingers.

Let’s take some of the tasks the students have to do from the video:

54 * 51

Use cross multiplication:

50*50 =2500.

Add (4+1) * 50 = 250.

2500 + 250 = 2750.

Last: 4 * 1 = 4.

Answer 2754.

54/7

I have talked about division here on this forum a lot.

In this case remember that 7*7=49.

Subtract from 54 to get a remainder of 5.

Now the part after the decimal point:

50 => 7 times. 1 left. So next digit is 1.

We get as an answer: 7,71…

Mental calculators know that there is a short sequence of digits that repeat.

See this:

1/7 = 0.14285714285714285714285714285714…

2/7 = 0.28571428571428571428571428571428…

3/7 = 0.42857142857142857142857142857142…

4/7 = 0.57142857142857142857142857142857…

5/7 = 0.71428571428571428571428571428571…

6/7 = 0.85714285714285714285714285714285…

If the answer s 7.71… we know it repeats as follows:

7.7 142857 142857 142857 142857 …

99 * 98

See this as (100-1) X (100-2)

Do the cross:

100X100 = 10,000

(-1-2)X100 = -300

-1X-2 = 2

Add those 3 together: 9,7020

999 * 996

Same as above with 1000-1 & 1000-4

1003 * 996

Same as above with 1000+3 & 1000-4

43132 * 11

This is a trick that goes as follows:

4 (4+3) (3+1) (1+3) (3+2) 2

answer: 474452

You add the left and right digit.

244 * 125

I would do this as (244/8) * 1,000 = 30,5* 1000 = 30,500.

Multiplication of numbers between 11 and 19:

14

17 X

13

15 X

12

13 X

15

17 X

11

15 X

15

19 X

This is done by realizing that each task can be written as:

(10+a)*(10+b) = 100 + 10*(a+b) + a*b

Squares of numbers ending in 5 and 1.

45^2 = (4 * 5) | 25

The | denotes a hundred separator.

4*5 = 20.

So 2025

55^2. 6 * 5 = 30 => 3025

85^2. 8 * 9 = 72 => 7225

41^2 = 4^2 * 10^2 + 4 * 2 * 10 + 1^2 = 1600 + 80 + 1

51^2 = = 5^2 * 10^2 + 5 * 2 * 10 + 1^2 = 2500 + 100 + 1

81^2 = = 8^2 * 10^2 + 8 * 2 * 10 + 1^2 = 6400 + 160 + 1

41 X 49

These have the form (45-a)(45+a).

The answer is 45^2 - a^2

We just did 45^2 and the answer is 2025.

a=4, so a^2 = 16.

2,025-16 = 2,009.

46 X 54

These have the form (50-a)(50+a).

The answer is 50^2 - a^2

50^2 = 2500

a = 4. a^2 = 16

2,500-16 = 2,484

35 X 47

Could be done as 41^2 - 6^2

Or just cross multiplication.

45 X 63

Could be done as 54^2 - 9^2

Or just cross multiplication.

5453 X 999

I probably done as 5453 * 1000 - 5453 =

5,447,000 + (6,000 - 5,453).

If you know your complements you immediately know that 6,000 - 5453 = 547.

Answer: 5,447,547

If you see anything in the videos that you cannot understand, just ask.