Ninety three minus seven

When my doctor would come by for rounds in the hospital, he would ask me to answer arithmetic questions. The one that he usually asked was 93 minus 7. To that answer, I was to minus 7 again, and to that answer, minus 7 again, etc. I thought he was just playing a game with me. For a while there I couldn't get past 93 minus 7. Really, he was checking to see if a certain part of my brain was functioning properly.