Effect: You KNOW the effect; people like Kreskin have featured it on television and in their live concerts. An ordinary note-pad is introduced, and casually shown. It is opened, and the pages are riffled to prove its innocence. A spectator is asked to write a three-digit number in the pad. A second spectator and then a third is then asked to write two more three-digit numbers just beneath each other. At this point no-one knows what those three three-digit numbers are. A fourth spectator is asked to add the numbers up, writing the total beneath the three numbers. No-one knows what this total is at this point. The performer's assistant brings out a large card, white-board, etc. with a prediction on it; alternatively, the performer calls attention to a prediction which was sitting in plain view the whole time. A spectator shows and calls out the total of the three random numbers written on the pad...IT MATCHES YOUR PREDICTION!
NOTE: A mostly ordinary pad is used. The gaff is completely undetectable. almost completely self-working, so you can focus on presentation and ENTERTAINING your audience! Can be used in conjunction with other "revelation" or prediction-type effects: have one spectator write a number, another a suit, the third spectator calls out the full name of the card on the pad which matches the mentalist's prediction. Lends itself to many more possibilities.