Mentalist trick using mathematics

 

52 cards

2 piles

26 each

then remove a pile (x cards) the isolated pile (pile 3)

A selection is made from pile 2 (out of 26 cards)

the selected card stays the bottom of pile 2

put pile 2 on top of pile 1 

the selected card is then 26th 

then strip the bottom and top cards of the resulted pile

then move the resulted pile

count the number of cards in pile 3

then take the resulted pile and count the same number of cards Y

the selected card will be at the same number Y

The math behind this trick is:

once the cut x cards (pile 3) 

you're left with their pile of 26-x cards, 

their selection and 25 cards on top. 

There will always be less cards under their selection than on top of it. 

By dealing one from top and one from bottom you just get rid of 26-x cards from bottom and top to a point where you're left with pile with their selection on bottom and x-1 cards on top

Another way to explain it:

Amount of cards after shuffling: 26 (pile 1)+(26 (pile 2)-x)

Position of selection: Amount-(26-x)*2

=26+(26-x)-(26-x)*2

=26+26-x-52+2x

=x

(x is the amount of cards the spectator has cut)

Tada!