If you use the Rubik's Cube and the cards in excellent ending for any routine, if you don't use the Cube you can still emphasize the fact that the choice of the number will be random a bit as it is random when one shuffles the Rubik's Cube, so many possibilities.
A card chosen and left in the middle of the deck.
A rubik's cube drawn on one of your business cards and a number on each square.
The spectator will have to randomly select 3 numbers which will add up.
And just that random number will be the number where the selected card will be found.
And as a final climax, the number chosen will be written behind the card!
THE ONLY CARD WITH A NUMBER BEHIND.
A true, totally automatic and immediately resettable miracle.
You can leave both your ticket and card to the spectator!