# This Incredibly Simple Math Trick Is Blowing People's Minds on Twitter – ScienceAlert

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. 🤔

I’m really quite cross that this was NEVER taught in my maths class.

how am I today years old to learn this. All the useless math taught at school & discovered this 30 years after leaving!!

Absolutely stealing this for my next science lesson! 😂👍🏻

I wish I’d seen this thread when I was about ten years old.

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.

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!

Whoa…I teach Maths at primary level and had never realised this.
50% blown away/50% going DOH! pic.twitter.com/4yhxgFSgDq