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One Basket or Group Photo

Libby Jared helped to set up NRICH and this is one of her favourite problems. It's a problem suitable for a wide age range and best tackled practically.

Below 400

Stage: 4 Short Challenge Level: Challenge Level:1
See all short problems arranged by curriculum topic in the short problems collection

If the pattern below is continued, what number will appear directly below 400?

    2 3 4    
  5 6 7 8 9  
10 11 12 13 14 15
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.