The Calendar Trick

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.

&nbsp. 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 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.

Here's this months calendar for you to practise on!

January 2008
Su	M	Tu	W	Th	F	Sa
		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

And if you don't want to use a calendar....

These tricks work with ANY grid of numbered boxes, just as long as the numbers are continuous. Try them both on this grid:

13 14 15 16 17 18 19 20 21 22 23
24 25 26 27 28 29 30 31 32 33 34
35 36 37 38 39 40 41 42 43 44 45
46 47 48 49 50 51 52 53 54 55 56
57 58 59 60 61 62 63 64 65 66 67
68 69 70 71 72 73 74 75 76 77 78