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Put the nine numbers below onto a 3 by 3 grid so that each number occupies one space, and the product of every row, column and diagonal is equal to 1.
$$1 2 3 6 \frac{1}{6} \frac{1}{3} \frac{1}{2} \frac{2}{3} \frac{3}{2}$$
If you liked this problem, here is an NRICH task which challenges you to use similar mathematical ideas.