This trick makes you look like you've got the most awesome mental power. All you need is a calendar which has the dates lined up under the days of the week, so the numbers are arranged something like this:
. . 1 2 3 4 5
6 7 8 9 10 11 12
13 14 15 16 17 18 19
20 21 22 23 24 25 26
27 28 29 30 31
The "9 number" trick
- Ask a friend to draw a 3x3 box around ANY nine of the numbers on the calendar.
- Almost immediately you can say what the nine numbers all add up to!
- See how long it takes your friend to check the answer on a calculator. (Yawn...snore...zzzz...)
For instance, let's say the red numbers are chosen.
. . 1 2 3 4 5
6 7 8 9 10 11 12
13
14 15 16 17 18 19
20
21 22 23 24 25 26
27
28 29 30 31
They add up to 198.
THE SECRET: All you do is look at the number in the middle and multiply it by 9.
In this case the middle number is 22 and 22 x 9 = 198.
HANDY TIP: To multiply by 9 quickly, just multiply by 10 then subtract your number.
So to get 22 x 9 you multiply 22 x 10 =220 (easy!) and then subtract 22. With a bit of practise you can do this quickly in your head.
The "20 number" trick
- Ask a friend to draw a 5x4 box around ANY twenty of the numbers on the calendar.
- Almost immediately you can say what all twenty numbers add up to!
For instance, let's say the yellow numbers are chosen.
. 1
2 3 4 5 6 7 8
9 10 11 12 1314 15
16 17 18 19 20 21 22
23 24 25 26 27 28 27 28 29 30 31
They add up to 290.
THE SECRET: Add together the smallest and the largest numbers in the group. Multiply the answer by 10.
In this case the smallest number is 2 and the largest is 27, so 2 + 27 = 29. Then 29 x 10 = 290.
With the "20 number trick" if the calendar month is February occasionally it will not be possible to put a box round 20 numbers.