Bob Hummer's Lite-Ning Addition

Here's the effect
You hand the deck to a spectator for shuffling. Next, your helper is told to fan or otherwise spread the cards with the faces toward himself and make a free choice between either an ace or deuce. Whichever card he chooses, he is told to remove it from the deck and place it face down on the table. Now he is told to remove either a three or four and also place it face down on the table. This process is repeated with the spectator choosing a five or six, seven or eight, nine or ten, and finally, a jack (11) or queen (12). None of the spectator’s choices are influenced in any manner whatsoever.

The moment the spectator has removed the final card of the six selected; you correctly announce the total number that the cards add up to. What’s more, you can repeat the effect immediately and get a different total. The cards are not arranged in any way.

Secret
While the deck is an ordinary one, it has undergone some minor preparation beforehand. Using an X-acto knife, ALL OF THE EVEN CARDS ARE MARKED ON THE BACK by scratching away a small diagonal line in the border at the upper left and lower right corners of each even card. See the picture at right. Only the even cards (2, 4, 6, 8, 10, Q) are marked in this manner. Don’t worry about anyone catching on. Not even the closest observer will notice the marks. Return the even cards to the deck and shuffle it well. You’re all set.

Performance
Hand the deck to someone to be shuffled. After this person is satisfied the cards are thoroughly mixed, instruct him to spread the faces of the cards toward himself. Now ask him to select either an ace or a deuce and without showing you or anyone else, to place it face down on the table. As already discussed, this is repeated with the 3-4, 5-6, 7-8, 9-10, and J-Q. Briefly mention that the value of a jack is 11 and the queen, 12; when you get to that point. Unknown to the spectator, you are SECRETLY COUNTING HOW MANY EVEN CARDS are placed down. That’s it! Don’t worry about the odd cards. Just count the even ones. Now here’s the clever bit, if all of the selections were odd, the six cards would total 36 – so that becomes your "base" number. Now, all you have to do is ADD "ONE" FOR EACH EVEN CARD to that number. Thus, if there is only one even card, the total will be 37 (36 + 1). Likewise, if there are only two even cards, the total will be 38 (36 + 2), and so on, and so forth.

The nice thing about this trick is that it bears repetition with a different total.