Wipe Out

Years 3 - 6

Summary

By subtracting one number only from the display, children are asked to wipe out one or more digits from the display. This seems simple to teachers, but the examples below show that the place value challenges involved are not necessarily straightforward for students. Suitable for threading.

Materials

  • One calculator per child
Note: This investigation has been included in Maths At Home. In this form it has fresh context and purpose and, in some cases, additional resources. Maths At Home activity plans encourage independent investigation through guided 'homework', or, for the teacher, can be an outline of a class investigation.
  • Visit the Home Page for more Background.
  • For this specific activity click the Learners link and on that page use Ctrl F (Cmd F on Mac) to search the task name.

Procedure

Introduce the activity to the class by writing a 3 digit number on the board, eg: 352
  • What is this number?
  • Insist on an answer in place value form (eg: three hundred and fifty-two) rather than just a statement of the digits (three-five-two or three fifty-two).
  • Ask the children to write the number on their calculator.
  • Your challenge is to make your screen become 302 by taking away one number.
  • As you say this write the 302 under the 352, draw an arrow between them and write 'take away ONE number'.
  • You may also have to explain that by one number you don't necessarily mean one digit.
As the children discover they need to subtract 50 in order to successfully wipe out the 5, they hold their 'successful' calculator in the air to invite you to see them do it. You will easily be able to get around most of the class before needing to write up the next example.

Demonstrate how to write what has been done as a subtraction and ask students to record it. Ask a student to set the next challenge for the class.
Later, ask the children to challenge each other.

  

Content

Listed alphabetically.
Primary content in bold.
  • complementary addition
  • mathematical conversation
  • place value
  • problem solving
  • recording - calculator
  • recording - written
  • subtraction

From The Classroom

What the Children Said
Kerry Wode, St. Thomas the Apostle, Kambah

Just a few snatches of conversation overheard while the children were working with their partner. Isn't asking questions the work of a mathematician?

Challenge: Type in 5647. Using addition, turn the 7 into 2?
  • Can I make other numbers change?
Challenge: Type in 5647. Using subtraction, turn the 4 into 7.
  • I don't have enough tens? Where can I get some? (trading!)
Investigation:
  • Can I make the new digit a different way?
Sure. Record some of your ways.
Story 1
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!!

  Story 2
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:

Start: 328,756
Target: -328,756
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!

Variations

  1. Extend to 4, 5, 6... digit numbers.
  2. Wipe Out two or more columns, eg:
    Start ... 5278
    Target ... 5008
  3. Make the Target Number 1, 10, 100, 1000 etc.
  4. Draw up a table like this to start the challenges of the day:

    Start 394 156 488 890 4,689
    Target  94 106 480 800 4,009

  5. Encourage mental arithmetic with Wipe Outs like:
    Start ... 467
    Target ... 396


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