I failed on this problem, and so far I've failed on 1117 other problems. I've also succeeded at 959. My percentage is under 50% because I am sometimes unfocused, and because success always brings harder problems.
I've honed my skills through nearly fifteen hours on Chess.com's Tactics Trainer since late January. This effort has displaced my Tactics Training Plus as a priority. For those of you following my Resolutions, a kind of failure is in evidence. I have been consistent in spending ninety minutes per week with tactics training, but the form differs from the plan that I had resolved to follow. I will be returning to training against Hiarcs and other silicon beasts from the positions in Reinfeld, but I think it is good to vary the methods of training from time to time.
Chess.com is an excellent chess site. It is the newest member of a small group of top class turn-based sites, joining ChessWorld, Red Hot Pawn, and Game Knot. The other dozen or so sites at which I've played have their merits, but cannot compete as equals with these four. The quality of Chess.com's tactics trainer is matched by a similar feature at ChessWorld, but is missing or no where nearly as good at the other two. Of the four in the top class, two stand above. ChessWorld was created in 2001; Chess.com arrived in 2007.