Sandy Drycus, Leadville, Colorado
During the activity Wipe Out, two girls were playing with the number 756. I asked them if they could turn 756 into 56. After thinking for a moment, one girl confidently replied:
Oh yes, I just need to take away the 7.
She quickly subtracted 7 from 756.
Hey wait a minute. The number is wrong.
What did you get?, I asked her.
I got 749. But look...
She erased 749 and typed back in the number 756.
She put her finger over the 7 on the calculator window.
Now it says 56. All I need to do is take away the 7, but when I did that I got a different number.
I asked, Yes, I saw that number on your calculator. Why do you think you got that number when you subtracted 7 from 756?
I think my calculator is broke.
Okay. Let's try it again with another calculator.
She then told her partner, who was equally baffled, to type in 756. Again, the girls subtracted 7 and this calculator also gave them the answer 749.
Both girls looked at me and said Why is this happening?
I replied. Let's type 756 back into your calculators. Now read the number to me.
With that hint, the girls read the number over a few times saying, seven hundred, fifty-six.
Suddenly, one girl got so excited she could hardly hit the keys on the calculator as she typed in minus 700. It was truly an 'aha' moment for them when 56 appeared in the window.
The number isn't 7 it is 700. I get it!!
Grade 5, Clayton Elementary, Denver, Colorado
The class was just learning the game for the first time. The numbers were getting into six digits when one lad surprised the teacher by challenging the class to:
The second surprise was that most students were willing to accept the challenge.
The third surprise was that it wasn't long before someone else suggested that you only needed to double the Start number and then take that number away from the Start number!
- Extend to 4, 5, 6... digit numbers.
- Wipe Out two or more columns, eg:
Start ... 5278
Target ... 5008
- Make the Target Number 1, 10, 100, 1000 etc.
- Draw up a table like this to start the challenges of the day:
- Encourage mental arithmetic with Wipe Outs like:
Start ... 467
Target ... 396