Become a Human Calculator! Have "victim" write a random 5-digit #. Then you write number under theirs, making sure each digit of theirs + each of yours = 9 (<--but don't tell them this!); have them do it again; Again, match your digit to theirs to = 9. Have them finish w/ random #. To get answer: mentally subtract 2 from the last digit of their number; that becomes the last # in your answer. Then put 2 at the beginning of your answer then copy their random number (but make sure the last number is -2). So, if their last random number is 23789, your answer would be 223787. Have your "victim" check it using calculator.
Math Magic Trick #4 ... Well, that's pretty dicey! (Even though "die" is one and "dice" is more than one, with your permission, I will use the word "dice" throughout this explanation):
You need 2 dice for this trick. Have your "victim" roll dice and stack them while your back is turned. Turn to ask them if they are ready and assure them you are only checking to see that their dice are stacked (really, you are looking to see the value of the top dice). Ask them to mentally add all dice that are touching, i.e., the one that is on the table top, and the 2 that are stacked against each other. Meanwhile, while your victim is adding up these numbers, you take the number that is showing on top of their dice and SUBTRACT IT FROM 14. So... if their top dice has a 5, your answer is 9. If their top dice is 7, your answer is 7. Good luck! Have some fun!
P.S. If you want to try this w/ 3 dice, SUBTRACT FROM 21. Works every time!