MATHEMAGIC 24: CARD TRICKS

When the magician shows you 26 cards at the beginning of this trick
he only has to remember which card is on position 7.

When he puts the 26 remaining cards on top, this card will be on position 33

and  (10  a + 1) + (10 – b + 1) + (10 – c + 1) + (a + b + c) = 33.

A drawing that I made in 1976 to illustrate a wel-known priciple.