Lai Lai Lai! everybody play game lai!
1. think of a 4-digit number made up of digits from 0-9. repetition allowed but cannot be all same digit e.g. cannot be 1111, 2222, etc. but can be 1234, 0112, etc.
2. jumble up (i.e. permute) the digits to form another 4 digit number (0 allowed in first few positions e.g. 1240 jumble up become 0124 is ok)
3. subtract smaller number from bigger number to form another 4 digit number (again, 0 in front is allowed)
4. choose one of the 4 digits in step 3 and hide it, (but CANNOT hide 0), tell me the remaining 3 and i will tell you the digit you're hiding!
Lai Lai Lai, play play!