Tells to the person you are going to perform the trick: “I’m a human calculator! I can add five three-digit numbers even before you can enter them into the calculator!”. Says to the person to write on a paper the three three-digit numbers.

For example, suppose that the three numbers are:

121

437

264

When finished, ask to the person to show you the numbers. Now you add two more numbers to the list:

878

562

These two numbers, although they may seem randomly chosen, are an important feature for the trick. The sum of the first number with the fourth number and the sum of the second number with the fifth number must be equal to 999.

Thus, for this example, to calculate the sum of the five numbers just do: $2000 + 264 – 2 = 2262$.

Note that the addition depends always of the third number.