How would you do this mentalism trick?

[griff1960]

You have a large crowd before you and you ask everyone to mentally choose a number between 1 and 10. You then ask for three volunteers to come to the stage. You show them a folded piece of paper in your hand and tell them it is a prediction.

Each of them write their number on a large piece of paper. When all three numbers are added up they equal the prewritten number that you have in your hand.

 

[RealityOne]

We don't expose methods on the forums. Multiple answers can easily be found in Anneman's Practical Mental Magic or in Corindina's Thirteen Steps to Mentalism.