Can anyone recommend some good combinatorics problem, a single problem per person will also be appreciated, make sure to use problems from platforms like (CodeChef, Codeforces, HackerEarth, HackerRank, and Atcoder) only.
any suggestion or problem recommendation is appreciated.
Bro just apply tag combinatorial and u will see lot of problems , also here a blog which contain many problems on different tag.
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Or the same approach of @ssrivastava990, but on Codechef:
search for editorials with the tag combinatorics in discuss
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It contains good amount of spoj problems, what to do when stuck on spoj as it has no editorial section and discuss section is just grabage.
thanks man 
Try to google , many CF blogs and quora have solutions , and if not then ask here.