Have your spectator choose a three-digit number wherein the first and last digit are two apart. The number 301 would be an example, because 3 - 1 = 2. We will use 301 for this example.
Have the spectator think of the reverse of his chosen number. If he chose 301, the reverse (backwards) would be 103.
Have him subtract the smaller number from the larger one. In this case, 301 - 103 = 198.
Make sure he does not tell you any of the results and that you appear to be reading his thoughts and facial ticks.
Tell him to find the reverse of his new number. If it was 198, the reverse is 891.
Have him add the new number and its reverse (198 + 891 = 1089).
Tell him to think of his new number. Pretend to read his thoughts and do your own calculations before revealing his number to be 1089.
This always works because if you choose a three-digit number where the first and third digit differ by two or more, subtract the reverse from the original, then add the reverse of that subtraction to itself, you will ALWAYS end up at 1089.