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
For instance, let's say the red numbers are chosen.
. . 1 2 3 4 5They add up to 198.
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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.
For instance, let's say the green numbers are chosen.
Thanks to Louise Lennartsson who spotted a mistake in the calendar grid on the left (which we've corrected). We estimate that over 50,000 people had already viewed this page before she saw it! |
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.
| 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 | |||||
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
Links:
The Murderous Maths main index page