Tuesday, March 10, 2009

EXTRA CREDIT 3

Here is the new extra credit project I'll be having my students work on this month. It's a project called "Regifting Robin." When you go to that link you'll be asked to choose a 2-digit number and then to subtract from that number its first digit and its second digit. FOR EXAMPLE, if you pick 25 you would subtract 2 and then you would subtract 5; the result is 18. What you do with this resulting number is look on a grid at that link, and Regifting Robin will read your mind by telling you what gift is in the box with the number you ended up with.

I've seen some chat rooms where this trick is discussed, and some people say, "Not bad! She got it right half the time." Um . . . sorry, that means those people didn't subtract correctly. This does work every time.

WHY?!

2 comments:

mlast77 said...

How does this work? Can I attend your math class?! :)

I got to your site by trying to figure this one out ... I'm totally puzzled.

Heidi said...

I don't dare give out the answer YET as I have not collected it from my students.

I'll try to give a hint without giving it away completely. It has to do with place value and properties of factors and multiples. Do it a number of time and see what numbers you come up with. What do they have in common?

Sure you can attend my math class!