Why do this
gives pupils chance to practise addition and
subtraction, but also gives opportunities for conjecturing, proving
Which numbers are on both the blue and red
What does this tell you about the other four
What have you tried so far?
Encourage learners to ask their own "What if ...?" questions
and to pursue answers to them.
It might help to have a copy of the circles printed out or
drawn on paper/a miniwhiteboard and for children to use
digit cards to move around in the different positions.