Either find an example of a group which is expressed as the union of two proper subgroups or prove that such a group cannot exist.
The best solution was submitted by 고성훈 (수리과학과 2018학번). Congratulations!
Here is his solution of problem 2020-02.
Other solutions were submitted by 구은한 (수리과학과 2019학번, +3), 김기수 (수리과학과 2018학번, +3), 김동률 (수리과학과 2015학번, +3), 박현영 (전기및전자공학부 2016학번, +3), 유찬진 (수리과학과 2015학번, +3), 이준호 (2016학번, +3), 장우영 (서울대 경제학과, +3).