Counting postcards
Nick Cook, School of Computing Science, Newcastle University
http://www.ncl.ac.uk/computing/outreach/resources/primary
Preamble
In this activity we look at how to do addition, one step at a time. The first
purpose of this activity is to see whether pupils can work out what the
instructions achieve. Then we can explore how addition is achieved, and, for
example, the largest number that can be added using the "computer".
Activity
Resources
 A set of 20 cards or counters
 Three numbered post boxes - boxes with lids, slots to post cards through
and a flag to show that the box has something in it. The boxes are
numbered 1 to 3. Box 3 must have two flags – one on the left and one on
the right.
 Players: 2 number jacks, at least 1 postman Pat and at least 1 postwoman
Pat, 1 black and white cat, 1 shopkeeper at the post office counter
 Instructions for the number jacks, postman Pat, postwoman Pat, the black
and white cat, and the shop keeper at the Post Office counter
The boxes are arranged as follows, with flags down to start. Each number jack is
given 10 cards.
number
jack
number
jack
1
2
3
shopkeeper
Workshop worksheet (po2)
What does this "computer" do?
What is the largest number we can add up to with the set of resources?
What do we have to change to add to a larger number?
Does this tell us anything about computers and numbers?
What if we want to add 3 numbers together? Can we use this computer?
What happens if we tell a number jack to choose a number between 0 and 10
instead of 1 and 10? How can we change the instructions so that the computer
still works?
What if we want to do subtraction? What changes do we have to make? (Hint:
you need at least one more box)
Why does box 3 need two flags?
What happens if someone forgets to put a flag up or down?
Please discuss the suitability of this activity, improvements and extensions. For
example:
 Is the level appropriate to Key Stage 1 and which year?
 Are there changes you would suggest?
 Are there links to or uses in other parts of the Key Stage 1 curriculum?
 Are there links to your own practice?
Instructions for the number
jack
 Pick a number between 1 and 10.
 Write your number down.
 Count out that number of cards.
 When the flag on the post box in front of
you is down:
 Post all of the cards you counted out into
the post box
 Put the flag up on the post box
Instructions for Postman Pat
When the flag on post box 1 is up:
1. Open the lid of post box 1
2. Take 1 card out of post box 1
3. If post box 1 is now empty, put the flag
down on post box 1
4. Close the lid of post box 1
5. Post the card you took from post box 1 in
post box 3
6. If post box 1 is now empty, put the flag
on the right hand side of post box 3 up
Repeat the instructions.
Instructions for Postwoman Pat
When the flag on post box 2 is up:
1. Open the lid of post box 2
2. Take 1 card out of post box 2
3. If post box 2 is now empty, put the flag
down on post box 2
4. Close the lid of post box 2
5. Post the card you took from post box 2 in
post box 3
6. If post box 2 is now empty, put the flag
on the right hand side of post box 3 up
Repeat the instructions.
Instructions for the Black and
White Cat
When both flags on post box 3 are up:
1. Open the lid of post box 3
2. Take all of the cards out of the post
box
3. Give the cards to the shopkeeper at the
Post Office counter
4. Put both flags on post box 3 down
Repeat the instructions.
Instructions for the shopkeeper
Count the cards you have been given and write
the total on the whiteboard.