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Second Round, March, 2019

Combinatorics Difficulty 5.9 AIME, harder Prove it Netherlands

In a tall hat there are one hundred notes, numbered from 11 to 100100. 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 1010, 1515, and 2020 would be suitable (as 10<15+2010 < 15 + 20, 15<10+2015 < 10 + 20, and 20<10+1520 < 10 + 15), but the notes numbered 33, 44, and 77 would not (as 77 is not smaller than 3+43 + 4). 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

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Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.