MathDB

Problem 1

Part of 1990 Bulgaria National Olympiad

Problems(1)

removing parity indices from [1990]

Source: Bulgaria 1990 P1

6/9/2021
Consider the number obtained by writing the numbers 1,2,…,19901,2,\ldots,1990 one after another. In this number every digit on an even position is omitted; in the so obtained number, every digit on an odd position is omitted; then in the new number every digit on an even position is omitted, and so on. What will be the last remaining digit?
combinatorics