Following a few simple steps you will be able to instantly calculate the cube root of the spectator's number.
Ask the spectator to choose any whole number less than 100 and, using a calculator, to find its cube by multiplying the number by itself, then multiplying the answer by the original number.
Alternatively, if the spectator knows how to use scientific calculator, they can enter the original number, then press the x^y key followed by 3 and =.
The spectator then calls out the answer, and you instantly reveal the original number (i.e., the cube root).
Note that this method will NOT work with non-whole numbers, or with numbers greater than 99.
To master the system you must learn by heart the cubes of numbers 0 to 9, which are shown in the table below. You also need to consider the last digit of each cube.
Note how the last digit of the cubes for 0, 1, 4, 5, 6, and 9 end with the original number.
Note how the last digits for the cubes of 2 and 8 are swapped.
Note how the last digits for the cubes of 3 and 7 are swapped.
Ignore the last three digits of the number called out by the spectator and choose the memorised cube which is just lower (or equal) to the remaining number. The cube root of this is the first digit of your answer.
Now consider the last digit of the number called out by the spectator. This will indicate the last digit of your answer. For example, if the last digit of the number called out is 3, then the last digit of the cube root is 7 (see the last digit values in the table above).
Spectator calls 185193
Spectator calls 438976
Before you try this out on your friends, you should practice until you can calculate the cube root instantly and without error.
If you can't do this, then practice some more!
To practice and assess your ability, you can use the Cube Root Tester shown below.
Cube Root Tester