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'Below 400' printed from https://nrich.maths.org/
Note that the number at the end of the $n^\text{th}$ row is $n^2$, so $400$ will lie at the end of the $20^\text{th}$ row.
The row below will end in $21^2$, i.e., $441$, so the number directly below $400$ will be the number before $441$, which will be $440$.