Rating:

Easy reverse engineering challenge - solve this via observing the behaviour and brute forcing outputs.

https://advancedpersistentjest.com/2018/05/21/writeups-numbers-game-babyre-rctf-part-2/

Original writeup (https://advancedpersistentjest.com/2018/05/21/writeups-numbers-game-babyre-rctf-part-2/).