Problem Solution

Weekly Problem 42 - 2012

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.