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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 $440$.

*This problem is taken from the UKMT Mathematical Challenges.**View the archive of all weekly problems grouped by curriculum topic*