Question

a. Prove that among `11` natural numbers, it is always possible to choose two such that their units digit is the same.

b. Prove that among `11` natural numbers, it is always possible to choose two such that their difference is divisible by `10`.

Difficulty level (1 very easy - 10 very hard): 2

Topics:
Combinatorics -> Pigeonhole Principle Number Theory -> Modular Arithmetic / Remainder Arithmetic -> Divisibility Rules -> Divisibility Rules by 5 and 25
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