I posted this card trick video a few days ago and well, a few days ago, ElBandMonkey came and asked for hints on how the tricked worked. It’s rather simple to get once you see the pattern. In fact, this trick can be done with any odd number of rows and any odd number columns greater than 1. However, the number of questionings may differ depending on the number of rows and columns. Since you ask the person to tell you where the card is 3 times, he/she basically gave you enough information to figure out where the card is. It just so happens with the steps that you follow, it allows lands the card in #11 spot.
Here’s how it begins, with 21 cards:
0 1 2 3 4 5 6 7 8 9 A B C F D E G H I J K
Ask the person to select the column their card is in. Let’s say they chose column 3:
0 1 2
3 4 5
6 7 8
9 A B
C D E
F G H
I J K
Redeal the cards with the selected column in the middle. Note where the cards land.
0 3 6
9 C F
I 2 5
8 B E
H K 1
4 7 A
D G J
Notice that there’s only 3 cards max in each column that can be the correct card.
Ask the person to choose again and for this trial, lets take the worst case scenario: column 2.
0 3 6
9 C F
I 2 5
8 B E
H K 1
4 7 A
D G J
Redeal the cards with the selected column in the middle.
0 9 I
8 H 4
D 3 C
2 B K
7 G 6
F 5 E
1 A J
So no matter which column the person chooses now, you can guarantee it’d be the card smack in the middle of the column. And when you stick that column in the middle, you can guarantee the spot of their card to be #11 (right in the middle of 21 cards).