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If the pattern below 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.   

This problem is taken from the UKMT Mathematical Challenges.