Another Math Trick

We are working on another math trick in my math group! My students have not practiced it yet, but I've done it several times for them and we've had some lightbulbs go off. If you would like, you can encourage practicing this at home. As with the other math "tricks" we've done, the real purpose is to find fun and real-life ways to practice skills we are working on - in this case, addition of several numbers with renaming.

I haven't thought of a name for this one yet, and am happy to take suggestions! Here it is, at least partially explained. I'm wondering if any parent can tell me why it works! Have fun.

The Trick of No Name

Have someone give you a four-digit number that has no pattern or repeating digits (no 2626 or 5555, for instance), that is less than 9000.

5468, for example

Then add 20,000 and subtract 2 from that number secretly, and write this number on a piece of paper. In this case, the number that you write down is 25,466). Tell your audience that you have seen into the future, and know what number we will end up with.

Ask for another four-digit number with no pattern or repeating digits.

2346, for example

Then say, "Now it is my turn. Let’s see, I’ll pick:"

7653

Ask the audience for another choice, for example someone might suggest:

3490

Then say, "I’ll take the last turn…Hmmm, how about.."

6509

Then add all five numbers together. What will you get? Why, 25,466 of course!

Do you know how it works?