What is so hard about this IMO problem?

Over 90% of International Mathematical Olympiad (IMO) participants scored zero marks for this question. But the solution is beautifully simple.

Russell Lim

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An anti-Pascal pyramid is a finite set of numbers, placed in a triangle-shaped array so that the first row of the array contains one number, the second row contains two numbers, the third row contains three numbers and so on; and, except for the numbers in the bottom row, each number equals the absolute value of the difference…

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Russell Lim

I teach high school mathematics in Melbourne, Australia. I like thinking about interesting problems and learning new things.