Ok, try this one...
Make sure you have a full deck of cards(52)using aces as 1s and pictures as 10s.
Start by placing the first card face up on the table, whatever that face card is from that number you count to eleven. so for instance if your first card is a picture you start at 10 then just add a single card from the pack to make that total eleven, then turn that set over so it's face down, if ace is your 1st card you will have to add a further 10 cards on to make the total. repeat this until you have used all the cards or as near as, don't forget to turn all the sets over, making sure that the first card which would have been bottom card now comes to the top.
Get a friend... if you have one
to pick three of the sets, leave them three on the table and collect all the other cards up.
Now the clever bit, count the remaining cards(quietly)till you get to 16, then start the count again, counting the remaining cards.
Whatever the next total you come to is, that will be what the top three cards of the remaing three sets on the table add up to.
Have fun trying
Ps, forgive me if you end up banging your head against the wall. it's an easy trick to do but maybe harder to explain how to do it.