Imagine to have two regular decks of cards.
The spectator can freely choose from one deck, four random cards to get a random number and from the second deck a random card.
No control on the decks by the performer.
Despite this, the spectator's card will be found at the same number created with the sum of the values of the four cards selected from the first deck.

!Key Points!
Two regular deck of cards
Completely self working
No sleight of hand
No memory work
Reset in a minute