STEP 1
Ask a friend to write down a number (any number with more than 3 digits will do, but to save time and effort you might suggest a maximum of 8 digits).
Example: 83 972 105
STEP 2
Ask him to add the digits.
Example: 8+3+9+7+2+1+0+5 = 35
STEP 3
Ask him to subtract this number from the original one.
Example: 83 972 105—35 = 83 972 070
STEP 4
Ask him to select any digit from this new number and strike it out, without showing you.
Example: 83 972 070
STEP 5
Ask him to add the remaining digits and write down the answer he gets.
Example: 8+3+9+7+0+7+0 = 34
STEP 6
Ask him to tell you the number he gets (34) and you will tell him which number he struck out.
SOLUTION
The way you do this is to subtract the number he gives you from the next multiple of 9. The answer you get is the number he struck out.
Example: The next multiple of 9 here is 36 (9 x 4 =36)
36—34 = 2 and there you have your answer, isn’t it easy?
Note: If the number he gives you after step 5 is a multiple of 9, there are two possible answers. Then you simply tell him that this time he crossed out either a 9 or a zero.