(In reply to re: A Little Hocus Pocus
100 cards. Although it could be adapted for more.
The method I was taught (and used) involved - "A little hocus pocus" i.e. a trick. to get part of the answer.
When I was taught it was explained that a complicated method was needed to determine the numbers but that "the trick" made those "almost impossible mental calculations" unnecessary. This complicated method was not given to me.
Based on that information and the problem being from an exam paper I assumed the existance of a complicated solution.
Apologies if I confused. I eagerly await the official solution.