Library / /283 of 356

, 2010

Combinatorics Difficulty 2.0 Junior Find the answer Canada

How many different pairs $(m, n)$ can be formed using numbers from the list of integers $\{1, 2, 3, \ldots, 20\}$ such that $m < n$ and $m + n$ is even?

(A) $55$ (B) $90$ (C) $140$ (D) $110$ (E) $50$

Want a route through all this instead of an archive? The track puts 2,604 problems in a working order, from Junior Challenge level to the IMO shortlist.

Source: CEMC, University of Waterloo, licensed CC-BY-NC-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project. Solutions are the publisher's, linked not copied.