Problem A: Vasya and Password: https://youtu.be/5ZdRBNJXHjg
Problem B: Relatively Prime Pairs: https://youtu.be/vcF8mTjgt6o
Problem C: Vasya and Multisets: https://youtu.be/q56uTA6t9Uk?t=3m8s
Problem D: coming ...
Educational Codeforces Round 51 (Rated for Div. 2)
1051A - Vasya And Password
1051B - Relatively Prime Pairs
1051C - Vasya and Multisets