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# Crossover

##### Age 11 to 14 Short Challenge Level: The numbers $2$, $3$, $4$, $5$, $6$, $7$ and $8$ are placed in the squares in this diagram.

The four numbers in the horizontal row add up to $21$, and the four numbers in the vertical column also add up to $21$.

Which number should replace $x$?

If you liked this problem, here is an NRICH task that challenges you to use similar mathematical ideas.

This problem is taken from the UKMT Mathematical Challenges.
You can find more short problems, arranged by curriculum topic, in our short problems collection.