Math Olympiad for Elementary | 2019 | Division E | Contest 4 | MOEMS | 4C

Problem: The numbers 10, 11, 12, …, 98, 99 are written as one big 180-digit number (1011121314 … 979899). All the even digits are then removed. How many digits remain?

Key: Slowly removing the even digits from first the units place, then the tens.

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