What kind of problems are you thinking of? I don't know if you play chess, but that is some of the most complex problems I do, when I get the magazine Chess Life every month (I became a life member of the USCF back in the early 80s, so I continue to get the magazine!), I go through the problems they have every month, with the answers in the back. I do them in my head, and often they are quick, but sometimes in many moves, and I almost always figure them out, even the long ones. And I play most the games they give, start to finish, in my head. Helps keep my mind avtive!