It's not always easy to work out percentages in your head, but a simple maths trick being shared on Twitter can make it incredibly easy to perform difficult or unwieldy mathematical calculations on the spot.

Described as a "fascinating little life hack" by UK-based copywriter Ben Stephens, the trick involves doing a simple number swap to figure out your answer. It's not a new technique, but the huge reaction online shows lots of people never knew the workaround existed.

As Stephens explains, if you ever have to calculate a difficult percentage on the spot without pen and paper or a calculator, you can use a simple shortcut - flip the numbers around.

"So, for example, if you needed to work out 4% of 75 in your head, just flip it and do 75% of 4, which is easier," Stephens tweeted.

Indeed, if you swap the numbers and calculate 75 percent of 4, you get 3 – and 3 is the same answer when you calculate 4 percent of 75. (In case it doesn't click straight away, 75 percent is three quarters, so in this case it becomes a simple division problem.)

Another example: "18% of 50 feels hard to calculate," Stephens wrote. "But 50% of 18 is a doddle, right?"

This basic principle – simply stated as x% of y = y% of x – is probably well known to millions of people across the world.

But as the tweetstorm generated by Stephens' original comment shows, it's a mind-blowing discovery for many who were never made aware of this simple substitution, with one user saying the revelation "got me excited about math for the first time in decades":

Have a degree in Applied Mathematics and never knew that. ? — Kamal DJAIDAOUI ???????? (@DjaidaouiKamal) March 8, 2019

I'm really quite cross that this was NEVER taught in my maths class. — mellymelmelmel (@pikminlover) March 7, 2019

how am I today years old to learn this. All the useless math taught at school & discovered this 30 years after leaving!! — Robin Hartley (@RobinHartley2) March 8, 2019

Absolutely stealing this for my next science lesson! ??? — Emma Gill (@EmmaGill15) March 6, 2019

I wish I'd seen this thread when I was about ten years old. — Roger O'Keeffe (@rogerokeeffe) March 5, 2019

This is one of those weird things where, now that you've pointed it out, it's so obvious that I wonder why I never knew it before. — Safwan (@zinjabil) March 6, 2019

This is so cool... have a-level in maths and been coding for over 20 years... this will be so useful... don't be deterred by people dissing it! — James Stone (@mimeartist) March 6, 2019

Whoa...I teach Maths at primary level and had never realised this.

50% blown away/50% going DOH! pic.twitter.com/4yhxgFSgDq — Magical Maths Notts (@MathsNotts) March 6, 2019

Others said they knew about this 'trick' all along, having been taught it in school.

Really, is this new? I always thought that's the way to do it since being taught that in primary school in India 35 years ago. That's the only way I've known. And I thought the whole world knew it. — Nindy (@Happy_Andy) March 7, 2019

As some people point out, though, the technique doesn't always necessarily work (in terms of making things simpler, at least).

For example, using the substitution with some calculations, like trying to calculate 17% of 12, would just give you 12% of 17 (which really isn't any easier to do in your head).

Also, if the numbers get larger or start involving decimals (or other complications), it won't make percentages much easier to calculate.

But as long as you're dealing with fairly small numbers, it's a real help for some of us.

If you even call it living. — Ben Stephens (@stephens_ben) March 5, 2019

Enough said!