Math Prediction Trick Tutorial

This is a math trick that is very easy to do, but looks incredible. The magician makes a prediction and then writes a 4 digit number. Then he asks the spectator for a random 4 digit number. He writes another number and then asks for another. After 5 random 4 digit number are written, he adds them up and shows that the new number matches his prediction.

This is easy enough for a beginner to do, but looks like professional magic.