Pull a 3 from the deck and lay it face down. Now have the participant pick any card from the deck. Have them double the value of the card and then add 2. Have them multiply this number by 5. Finally, subtract 7 from the result. In the example below, the person would have picked the 8.

♠

8

8

♥

3

3

Here's how it works:

[(2*x*+2)5]-7

[(2(8)+2)5]-7 = [(16+2)5]-7 = [(18)5]-7 = 90-7= 83

They picked the 8 and you picked the 3. Using the mathematical formula will always yield the 3 so you have to pick the 3. The math will tell you the rest!

Thanks to http://www.videojug.com/film/how-to-do-a-maths-magic-trick.