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Second Round, March 2019

Netherlands 2019 counting and probability

Problem

In a tall hat there are one hundred notes, numbered from to . You want to have three notes with the property that each of the three numbers is smaller than the sum of the other two. For example, the three notes numbered , , and would be suitable (as , , and ), but the notes numbered , , and would not (as is not smaller than ). You may (without looking at the numbers on the notes) take some notes from the hat. What is the smallest number of notes you have to take to be sure to have three notes that meet your wish?
Solution
11
Final answer
11

Techniques

Coloring schemes, extremal argumentsRecurrence relations