Saturday, September 6, 2014

1089 Math Magic Trick, The Good Girl (Book Club), Apple Pie Cookies


It's incredible to me that I learned this math trick over 25 years ago. I was at a workshop for new teachers at my alma mater, Rutgers University, ( the summer between graduation and my first year teaching. I do not remember much other than being very nervous about the start of teaching, but what did stand out to me is this one magic trick that I learned. I have done it every year, and kids are really cute about it. Here is how it goes.

I tell the students I am going to perform a math magic trick for them. I ask for a volunteer to go to the board, and I stand in the back of the classroom and face the other direction. I tell them to:

1. Pick a 3-digit number whose numbers are in descending order, i.e., 531, and to write it on the board.

2. Reverse the digits, write that number underneath the original number, and subtract.

3. Reverse the digits in the answer, and add that to the answer from step 2. 

I make sure that students are in agreement that the answer is correct. I ask another volunteer to write the answer on a piece of paper and crumple it up. I then ask the first student to erase the board.

I tell the students we are going to go outside, but I need to grab my lighter first. This sparks some attention and excitement. We find a good place outside, and I put the paper on the ground (a pie tin works better) and light it with the lighter. After a minute or two, when the paper is done burning and students are curious about what is going to happen, I tell them I will see the answer to the problem in the ashes.

I look in the ashes when the burning dies down, grab them, and wipe them on my arm. The answer of 1089 "miraculously" shows up on my arm.

After the initial shock, students come to the conclusion that the answer must 1089 every time. Here is a simple proof. Here is a more involved proof. I like to do the more involved proof when I teach 10t + u and the reversal of digits in Algebra 2 Honors, but all of my classes definitely enjoy it.

I almost forgot to tell how I got the number on my arm...a good magician never tells her secrets, but I could not help but ask the demonstrator how he did it back when I was 20 years old, so I know the kids are very curious about how it is done! The answer is to just take liquid soap and write 1089 on your arm before sure to wear short sleeves and let it air dry first!


I am in a book club with some teachers from my school. We don't meet terribly often, but it is definitely great fun to get together with a mission and to talk with these wonderfully bright women about something other than school work. Our book this time is The Good Girl by Mary Kubica. If you liked Gone Girl, you will love this book.


Fall is definitely a time for apple recipes! I recently made apple pie cookies from
They were very messy but very fun to make. I had no idea how to do lattice work for a pie, so I found that here. I also made a Kale salad with reduced apple cider dressing from a Publix Apron's demo class I recently took. Just a quick note: instead of using anise star, the chefs used an anise star tea bag and then never had to worry about the cheesecloth...just remove the tea bag and bay leaf before putting in blender. The walnut oil mixed with the cider made for an amazing dressing...I forgot to take a picture!

Have a great week,


bubblefootball said...

Great tricks shared,..
halifax magician

DimP said...

I Can Find the Number you Thought! The magic number!!!
Try it…!!!

MMS Info said...

It's mind blowing to me that I realized this math trap more than 25 years prior. I was at a workshop for new educators at my place of graduation, Rutgers University, the mid year in the middle of graduation and my first year instructing. I don't recall much other than being extremely anxious about the begin of educating, however what did emerge to me is this one enchantment trap that I learned. dubai magician