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Dutch Mathematical Olympiad

Number theory Difficulty 5.2 AIME, harder Find the answer Netherlands

For the second round of the Mathematical Olympiad, 999 students are invited. Melanie creates invitation letters in order of participant number: 1,2,3,…1, 2, 3, \ldots. For some values of n≥100n \geq 100, she notices the following: the number of participant numbers from 11 up to and including nn ending in a 55 is exactly equal to the integer formed by the last two digits of nn.
For how many values of nn (with 100≤n<1000100 \leq n < 1000) does this hold?

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

Solution

9

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