Years K - 2

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Estimating number is at the centre of this activity which is complemented by the Members activity Buttons. All the red plugs are placed in a bag and children try to withdraw a number of plugs which they first specify. The task is structured to support the development of subitising and mental arithmetic related to addition, subtraction, difference and complementary addition. Lots of variations and extensions are possible. Suitable for threading.


  • One Poly Plug set per pair
  • One calculator per child
  • One bag, box or tub


This activity was designed by Wendy Batchelor, Ruth Court, Dione Parker, Julie Jacobs, Kingston Primary School.


  • Remove all the plugs from a red board and put them in the bag.
  • The player thinks of the number of red plugs they want to withdraw from the bag and records it on their calculator.
  • The player then reaches in and tries to grab that many in one handful. Now they:
    • count the plugs
    • compare with the target
    • write or draw in their journal to explain the guess and the result
  • then replace the plugs and the other player tries.
  • Repeat the process as often as interest allows with a view getting as close to the calculator guess as possible.


  1. Children draw their own open number line (a line of any length with an arrow at each end) and record their guesses in one colour with marks above the line and in another colour with marks below the line.
  2. Use two or more boards and allow more than one dip into the bag with each hand, or one two-handed dip.
  3. When confidence has developed with red plugs, repeat the activity with the yellow/blue plugs. (What changes?)
  4. Students estimate the number they can grab with one hand, count the result and find the difference between the actual and the estimate. Record the difference (perhaps as a graph). Repeat the experiment and look for trends in the accuracy of estimates.


  • 1:1 correspondence
  • addition facts beyond 10
  • addition facts to 10
  • complementary addition
  • counting
  • data: collecting, recording, displaying
  • estimating number
  • number line - ordering, operations
  • numeral recognition
  • recording - calculator
  • recording - written
  • subitising
  • visual and kinaesthetic representation of number
  • writing numerals

Calculating Changes ... is a division of ... Mathematics Centre