Your written prediction will match a number selected at random.

Have someone select a three digit number with each digit different, such as 421. Under that number, have them write the number backwards and subtract the smaller number from the larger number (421-124). Now have them turn the resulting number around and add the two numbers together (297+792).

The final answer will always be 1089, which is the number you have written as a prediction! Remember, for this trick to work always use the zeros as shown in the second example.