Stage: 2 Challenge Level: 
Why do this problem?
This excellent problem
is so very good for number awareness, reinforcement and addition
and subtraction awareness.
Possible approach
Staring off in a very practical way with suitable rods would
be ideal in many circumstances.
Key questions
How did you get to this solution?
I see you've not got a (suppose - 9 when using 2,3 & 5 )
can you explain that?
Possible extension
What about four rods?
Which combinations work/not work and why?
Possible support
text
Addition & subtraction. Generalising. Multiplication & division. Working systematically. Place value. Trial and improvement. Investigations. Interactivities. Factors and multiples. Combinations.
Published December 2001.
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