The Big One
Years 2 - 8

A game for two players using the same calculator.


  • One calculator per pair (there's one on your phone)
  • Write the title of this challenge and today's date on a fresh page in your maths journal.

How To Play The Big One

  • Player A is the 'hider'. Player B is the 'finder'.
  • Player A thinks of a mystery number and 'hides' it in the calculator by entering [mystery number] = .
    Note: You may want to begin the game by limiting the choice to, say, 2-digit numbers.
  • If clear is not pressed, the calculator is now set up to divide by the mystery number whenever = is pressed.
  • Player B has to find the mystery number by entering [guess number] = .
    Note: Some calculators show a sign when [guess number] is pressed to remind about the operation which has been entered.
  • If the guess is correct, a special number (the Big 1) will appear on the screen. If the guess is not correct, it will provide information for the next guess.
  • Without pressing clear Player B continues to enter [guess number] = until the Big One appears.
  • Keep a record of each guess in your journal. It is also useful to comment on whether the guess was too big or small.
  • Score one point for each try.
  • Players swap roles. Winner is the player with the lower total score after, say, three (3) or five (5) rounds.

Player A pressed 3 = to hide the mystery number
and leave a zero (0) on the screen. Player B then
guessed 6 =. The calculator used the 3
stored in its 'brain' to get the answer 2.
Have fun exploring The Big One.

We suggest it is used at least three (3) times a week for about fifteen (15) minutes over several weeks.

Hunting For Whole Numbers

  • Teach the calculator to divide by 3, or 7 or any other number. Let's pretend you chose 5.
  • In your journal write today's date at the top of the page in the middle and write the calculator's number ( 5) under the date.
  • Put a line down your page for about 10cm.
  • Write DO at the top of the left column and DON'T at the top of the right column.
  • Your challenge is to find seven (7) numbers that give a whole number answer when you press [guess number] = .
  • When you find one, write it on the DO side like this.
    If you guessed 10 and the calculator's number was ( 5) the screen answer would be 2.
    In your journal on the DO side you write 10 5 = 2.
    Put a box around the 10.
  • If you make a guess that doesn't give a whole number answer you do the same thing on the DON'T side.
  • The aim is to find seven numbers that work without having any DON'T answers.

Hunting For Patterns

  • Teach your calculator to divide by any number you want. Keep it single digit to start with.
  • Now press [guess number] = and write the equation in your journal.
  • Do this 15 - 20 times. Each equation is a piece of data.
  • Organise your pieces of data to hunt for any patterns?
  • Perhaps there are some gaps in your data. What do you need to do?

Just Before You Finish

For this part you need your maths journal and your Working Like A Mathematician page.
  • Write at least five (5) ways you are working like a mathematician when you are The Big One activities.


Answers & Discussion

These notes were originally written for teachers. They have been shared from the Members section of Calculating Changes, which is a division of Mathematics Centre.

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Maths At Home is a division of Mathematics Centre