Math Magic Trick
Materials needed: paper, a dollar bill, pen or pencil for writing
1. Ask someone in the group for a dollar bill
2. Ask a child to write down the first 5 numbers of the dollar bill’s serial number
For Example: 5 6 7 8 9
3. Ask another child (or the same child) to rearrange the numbers in a different number
For Example: 7 9 6 5 8
4. Now, put the largest number on top and subtract the two numbers.
For Example:
-
79658
56789
22869
5. With this new number, you can have some fun now. Ideally, one of the presenters can do
all of this with the children while the other presenter walks away so that the other
presenter doesn’t hear or see any of the previous steps. When you’ve completed step 4,
you can bring the other presenter back to the group. Still, don’t show the results. Ask one
of the children to remove one of the numbers from the answer except for the number 9.
Tell the child to tell the other presenter the new number. For example, let’s pick the
number 8. So now, the new number is:
2269
6. The other presenter can tell the child what number is missing using something called
digital roots. We will take the remaining new number and add each digit together.
For example:
2 + 2 + 6 + 9 = 19; now, we will take this answer and add its digits
together until we have a one digit answer. Now, 19 – 1 + 9 = 10; 10 – 1 + 0 = 1; The digital
root is 1. Now, we will take this digital root and subtract it from 9: 9 – 1 = 8 (that’s the
missing number). This works every time except for the number 9. If the child picks a 9 or 0,
both answers result in 0. This works because we have a base 10 number system.
Usually, I do this a second time because everyone is blown away over how you are able to do
this. I try to do the math in my hand when I’m trying to figure out the missing number and
play around with the crowd. It’s fun.