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'Weekly Problem 9 - 2011' printed from http://nrich.maths.org/

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Barbara wants to place draughts on a $4\times 4$ board in such a way that the number of draughts in each row and in each column are all different (she may place more than one draught in a square, and a square may be empty). What is the smallest number of draughts that she would need?
 
 
 
 
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This problem is taken from the UKMT Mathematical Challenges.

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