Learn how to mentally calculate cube roots and amaze your friends!

Following a few simple steps you will be able to instantly calculate the

cube rootof the spectator's freely-chosen 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.

Note that this method will NOT work with non-whole numbers, or with numbers greater than 99.

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).

To master the system you must

learn by heartthe cubes of numbers 0 to 9, which are shown in the table below. You also need to consider the last digit of each cube.

Number Cube Last Digit 0 0 0 1 1 1 2 8 8 3 27 7 4 64 4 5 125 5 6 216 6 7 343 3 8 512 2 9 729 9 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

threedigits of the number called out by the spectator and choose the memorised cube which isjust lower(or equal) to the remaining number. The cube root of this is thefirst digitof your answer.

Now consider the

last digitof the number called out by the spectator. This will indicate thelast digitof 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).

## Example 1

Spectator calls 185193

- Ignore last three digits = 185
- Lower (or equal) cube = 125
- Cube root of 125 is 5 = first digit of answer
- Last digit of spectator's call = 3
- From table of cubes (shown above), when the last digit of cube is 3, the last digit of the cube root is 7
- Therefore the cube root of 185193 is 57

## Example 2

Spectator calls 438976

- Ignore last three digits = 438
- Lower (or equal) cube = 343
- Cube root of 343 is 7 = first digit of answer
- Last digit of spectator's call = 6
- From table of cubes (shown above), when the last digit of cube is 6, the last digit of the cube root is also 6
- Therefore the cube root of 438976 is 76

## Further Examples

Called

CubeIgnore

Last

ThreeLower

(or =)

CubeFirst

Digit

(F)Last

Digit

of CallLast

Digit

(S)Cube

Root

= FS1728 1 1 1 8 2 12 21952 21 8 2 2 8 28 50653 50 27 3 3 7 37 117649 117 64 4 9 9 49 148877 148 125 5 7 3 53 216000 216 216 6 0 0 60 357911 357 343 7 1 1 71 636056 636 512 8 6 6 86 830584 830 729 9 4 4 94

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

## by Psychic Science

Exclusive Card Decks