I discovered this site which has 100,000 interactive tactics and mate problems to solve. In addition, it has endgame training and keeps track of your Elo rating using the Glicko system and other statistics. It is completely free.
If you fail to solve the problem your Elo rating is lowered and the problem’s Elo rating is raised. Like in a real rating the amount the Elo change depends on the difference between the two and the Rating Deviation parameter. As a new player your rating is set to 1500 and your RD to 350.
The best line was found using the Stockfish engine with 20 sec thinking time. However, the site’s internal engine that evaluates alternative solutions is much weaker but it's optimized for tactical positions; it's Elo rating is over 2000. VISIT IDEACHESS