Maths Problem Solving

First we drew a grid with 9 squares, 3 in each row and column.  I challenged the children to put 10 counters in every row and column, but only 10, no more and no less.
This was trickier than we thought.  There's was lots of collaborating and discussion, and revising of mistakes.  All children could solve the problem and some found more than one possible answer.