Domino Trick
by Charles Geer in the
Arithmetic Teacher May 1992
Give the following instructions:
1. Think of a domino and count the number of pips on the left side of that domino.
2. Multiply this number by 5.
3. Add 7.
4. Multiply by 2.
5. Add the number of pips on the right side of the domino.
6. Now tell me your answer and I will tell you the domino that you selected.
You will now tell or draw (this is more dramatic) the domino that any student selected.
(1) Mentally subtract 14 from the final answer.
(2) Your two-digit answer tells the pips on each side of the domino.
(3) The tens digit tells the number of pips on the left side,
and the ones digit tells the number of pips on the right side.
Why does this work?
1. Think of a domino and count the number of pips on the left side of that domino.
Let x = number of pips on the left side of the domino.
2. Multiply this number by 5.
You get 5x.
3. Add 7. You get 5x + 7.
4. Multiply by 2. You get 10x + 14.
5. Add the number of pips on the right side of the domino.
Let y = number of pips on right side, so you get 10x + 14 + y.
6. Now tell me your answer and I will tell you the domino that you selected.
Subtract 14 and you get 10x + y,
so the x is the tens digit and y is the ones digit.