Effect: Make a prediction and give it to a spectator to hold. Reach in your pocket and pull out a pocket full of change. Ask one spectator to pick a penny, one a nickel, one a dime and one a quarter. Ask another spectator to add the years of the coins they randomly selected. The total will match your prediction. The remaining coins are shown to have different years. If they had selected any of the other coins your prediction would not have matched.
Secret: Collect coins with the same year but make sure to use a different value year for each year. This is an unusual force. No matter which coin of any value they pick they will select get the same known year. Since you know the years in advance the prediction is easy.
Presentation: Place the force coins in your right pocket. Place a similar set of coins but with different years in your left pocket. Write your prediction and give it to a spectator to hold. Take the force coins out of your right pocket and ask four spectators to each pick a coin of different denomination. Ask another spectator to add the years of the coins and write down the total. While they are adding the coins simply replace the remaining force coins back in your right pocket.
Show that your prediction matches the total and take your bows.
Take the non force coins out of your left pocket and while placing them on the table saying "If you had picked any of these instead the prediction would not have matched. Now that's magic"