Allow anyone to choose six cards from a full pack. Tell him the
court cards count 10, and the other cards according to their pips.
Having made his selection, tell him to lay the chosen cards upon the
table face downwards, without allowing you to see them, and to place
upon each as many cards as pips are required to make 12. Whilst he
is doing so, you should be out of the room or blindfolded. On your
return he hands you the cards left over, and you have to tell the
total number of spots on the six bottom cards.
Suppose he had chosen 10, 6, 1, K, 3 and 7, which totals 37, now
on the 10, he would place two cards to make 12; on the 6, he would
place 6; and on the 1, 11 would be placed, and so on. On receiving
the remaining cards from him you pretend to be looking through them
carefully, but you simply want to know how many he has given you,
which in the above example would be 11. To this number you add 26,
which gives 37, the total spots required.
Should there not be enough cards left on hand to complete the six
heaps, you can ask him how many cards he is short of, and this
number, subtracted from 25, will give the total. It is better not to
allow the person to choose six cards right off at the beginning, but
for him to shuffle and cut the pack as he pleases, and to take the
cards as they come.