This is from an idea by Sam Schwartz marketed by Tannen's. Here's another solution, in our opinion way easier than the original. You show four long strips of laminated cardboard with numbers on both sides. A spectator is asked to shuffle these cards and lay them in any order next to each other. The combinations are endless. Yet, you can immediately reveal the sum of those numbers.
It takes you two seconds while the spectator has to add them either with a pen and paper or a calculator. No memory work. The result is immediate and easy.