This is yet another trick using nine times table.
Tell the user to get pencil and paper (this is needed since we are going to tell him to cross out any number from final answer). And one more thing; this trick is best suited to a four digit number.
- Ask him to write down any number (At least four digits long). e.g. 6534.
- Ask him to sum up the digits e.g. 6+5+3+4 = 18.
- Tell him to subtract the above resulting number (i.e. 18) from the original number (i.e.6534) as, 6534 – 18 = 6516.
- Tell the user to cross out any number (except zero (0) if any) from the above answer (Say, he crossed out 5, to get 6X16)
- Tell him to type the remaining digits (excluding the crossed out number, so like 616) in a text box in your application.
Once the remaining number is with you then guessing the missing number is simply a case of applying some tricky math steps.
Your application will sum up the digits typed by the user (as 6+1+6 = 13). Now with this number 13 all you have to do is tell your application to find out next nearest multiple of nine. In this case, 18 is the next nearest multiple of nine number after 13.
Now you are just one step away to find out missing number, the one that was crossed out. Tell your application to subtract the multiple of nine from the summed digits… so, 18 – 13 = 5 and this 5 is nothing but the crossed out number. Magic…Isn’t it?