It's simple and easy to perform almost self-working.
The magician starts with five cards asks the spectator to select any one card and keep it away, with the remaining four cards spectator takes four colored shapes and keeps them on each card randomly keeping the respective color uncovered.
When the spectator adds all four numbers the total of all four is already printed on the back of the 5th card.
Perform this as a mentalism effect. write or reveal the number even before starting the trick.
The best part is its Selfworking.