How Liam Delap is able to work out cube roots quickly and look like a maths genius, and the simple trick you can learn to do it yourself

Liam Delap has an interesting party trick that you wouldn't really expect a footballer to pull out of the bag.

The Chelsea forward has gone on video a few times showing off his bizarre mathematical ability to quickly calculate the cube roots of large numbers.

Delap showed it off again to TNT Sports earlier this week, with the clip going viral on social media as he gave an immediate answer to the question: "What is the cube root of 148,877?"

How to instantly work out cube roots in your head

For those of you who need the reminder... you know how a square number is a number multiplied by itself (e.g. 7 x 7 = 49)? A cube number is the result of multiplying a number by itself twice. So, for instance, 2 cubed is 8 because 2 x 2 x 2 = 8, while 5 cubed is 5 x 5 x 5 = 125.

The cube root is the same thing but in reverse - so the cube root of 8 is 2, and the cube root of 125 is 5. But how is Delap able to work out such big, intimidating numbers so quickly?

Delap's only condition is that the cube number you give him has to be the cube of a number between 1 and 100. He has been coy about how he does it, merely saying that it was a trick that his maths teacher taught him at school and which he has never forgotten.

Well... so were we, so we can tell you how it's done - and it's surprisingly simple as long as you're able to remember two very simple rules and ten numbers.

We'll use Delap's 148,877 as the example. The forward has pay attention to the number he is given, and used a trick to calculate the first digit of the answer (5), and then a trick to calculate the second digit (3), to give the answer '53'.

The first thing he pays attention to, is how many thousands it is - i.e. the bit of the number before the comma (or the word 'thousand' when spoken aloud). In this case, that's 148.

What Delap does now is think of the highest cube number that is lower than or equal to 148 and quickly calculate its cube root - a matter of simple memorisation.

The first ten cube numbers

• 1 cubed = 1
• 2 cubed = 8
• 3 cubed = 27
• 4 cubed = 64
• 5 cubed = 125
• 6 cubed = 216
• 7 cubed = 343
• 8 cubed = 512
• 9 cubed = 729
• 10 cubed = 1,000

In this case, the highest cube number lower than 148 is 125, which has a cube root of 5. So the first digit of the answer is 5 - i.e. it's fifty-something.

To get the second digit is even easier: all Delap really needs to do is pay attention to the final digit of the cube number.

That's because there is a relationship between the final digit of a cube number and the final digit of its cube root. A cube number ending in 0, 1, 4, 5, 6 or 9 will also have a cube root that ends in the same digit.

As for 2, 3, 7 and 8, you just need to remember that they all reverse order - so a cube number ending with 2 has a cube root ending in 8 (and vice versa), while a cube number ending in 3 has a cube root ending in 7 (and vice versa).

Final digits of cube numbers and their cube roots

• Cube number ends in 0 = cube root ends in 0
• Cube number ends in 1 = cube root ends in 1
• Cube number ends in 2 = cube root ends in 8
• Cube number ends in 3 = cube root ends in 7
• Cube number ends in 4 = cube root ends in 4
• Cube number ends in 5 = cube root ends in 5
• Cube number ends in 6 = cube root ends in 6
• Cube number ends in 7 = cube root ends in 3
• Cube number ends in 8 = cube root ends in 2
• Cube number ends in 9 = cube root ends in 9

So, Delap knows from the first step that cube root of 148,877 is fifty-something - and because the final digit of that cube number he has been given is 7, he knows that the 'something' has to be 3. The answer is therefore 53 - and he's worked it out instantly.

With a little bit of practice, you can do the same thing yourself within seconds - and all you need to do is memorise is the first ten cube numbers and their roots, and how the final digits relate to one another.

As a helpful tip, the calculation of the first part becomes even easier if you're able to nail down the second rule. That's because if you can remember that 343 is a cube number but can't quite remember what its cube root is, all you need to do is think that it ends in 3 - so the cube root must be 7.

Try that down the pub and bore your friends to death!