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'Three Ball Line Up' printed from http://nrich.maths.org/
Why do this problem?
is a good context for encouraging learners to work systematically. Then they will be able to convince someone else that they have found all the possibilities.
The interactivity will help children to get a feel for the problem.
If red landed in the middle, how could the blue and green fall?
Where else could red land if it wasn't in the middle?
Can you use this idea to find all the ways?
How will you remember the ways you have found so far?
Encourage children to use four balls/counters.
Pupils would benefit from having three differently-coloured counters to use while tackling this problem.