Grunge by Geni

Imagine, the magician asks the spectator to select any four of a kind, and the magician splits the deck into some piles of cards. The spectator chooses which pile to put their 4 aces on top. And surprisingly, the magician can figure out exactly how many cards are in all the piles that have the aces on top.
+Easy to do
+Borrowed deck

Let's learn it