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The number at the end of the $n$th row is $n^{2}$,
so $400$ will lie at the end of the $20$th row.

The row below will end in $21^{2}$, i.e. $441$

So the number directly below $400$ will be $440$.

See all short problems arranged by curriculum topic in the short problems collection
This problem is taken from the UKMT Mathematical Challenges.