If I understand the story correctly, his metric was effectively "what fraction of the remaining life in the man's body is expended in a day?"

Also, I thought the answer to the last question was very obvious. Granted, he took a fraction of the time alloted, but it still seems like they acted like that one was supposed to be by far the most difficult.

At first blush, it does sound rather difficult. And naturally, it appeared difficult to Lord Iron. If it were as difficult as it appeared, I imagine it would shock our suspensions of disbelief that the cambist should solve it in such a brief span.