Stage: 4 Challenge Level:
Part One : Work systematically - look at $1$ multiplying something,
then at $2$ multiplying the same number, then $3, 4$ and so
Part Two : If this wasn't possible to do, how could you show that
it was impossible?
Part Three : Start with a smaller version of the same kind of grid,
perhaps having only $1, 2$ and $4$ with which to make each factor.