1. u take a 4 digit number 2. subtract sum of its digits 3. delete any one digit from the result. 4. tell the sum of the remaining three Now, i can tell always the deleted digit with no of maximum possibilities:

The number trick works because subtracting the digit sum from a number makes the result divisible by 9. After deleting one digit and summing the remaining three, the missing digit can be deduced from the sum modulo 9. When the sum of three digits is 0, 9, or 18, there are exactly two possibilities for the deleted digit (0 or 9), making the maximum 2 possibilities.