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Try not to direct learners too much at this stage but make sure they understand that they can use any resources or equipment that they might find helpful. (You might want to have these sheets of blank triangles available should anyone request them: Word document or pdf).
Once you feel that most children have made progress and understand the problem well (this does not necessarily mean that they have found all the solutions), give out this (.doc or pdf). Suggest to the class that when they've finished or can't make any further progress, they should look at the sheet showing three approaches used by children working on this task. Pose the question, "What might each do next? Can you take each of their starting ideas and develop them into a solution?". It might be appropriate to read through each method as a whole class before giving pairs time to work on each one. Alternatively, you may prefer to allocate a particular starting point to each pair.
Allow at least fifteen minutes for a final discussion. Invite some pairs to explain how the three different methods might be continued. You may find that some members of the class used completely different approaches when they worked on the task to begin with, so ask them to share their methods too. You can then facilitate a discussion about the advantages and disadvantages of each. Which way would they choose to use if they were presented with a similar task in the future? Why?
(You may find that conversations arise about the number of different solutions for each total. Encourage children to articulate what they think is the same and what is different in this context. They might not all agree!)
Tell me about this approach. What do you think s/he was doing?
How do you think this will help to solve the problem?
What do you think s/he would have done next?