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If the pattern shown is continued, what number will appear directly below $400$?

$1$

$2$ $3$ $4$

$5$ $6$ $7$ $8$ $9$

$10$ $11$ $12$ $13$ $14$ $15$ $16$

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

See all short problems arranged by curriculum topic in the short problems collection

This problem is taken from the UKMT Mathematical Challenges.