this you can say as math magic trick .
here is the problem : ask the viewer to think of a 4 digit or more digit number (without showing it to you say 986568) ask the viewer to do sum of the digits ,then it becomes 42 (considering above number) subtract this difference from main number ,that becomes 986526 now i say that cut any digit from this new number , say 2 then the number left is 98656 then ask the viewer to show the result i.e. 98656 now you look at the result suddenly you answer that the digit you removed is 2 the trick is that you do sum of the digits that is 34 it is near a multiple of 9 i.e. 36 and difference between the two is 2 and you say the answer 2 what is the reason behind this trick!!!