# 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

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 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!
.       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 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.

### 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