# This maths hack taking the pain out of percentages is blowing people’s minds

Sometimes, just sometimes, Twitter is there for the good things in life. This is one of those times.

Fascinating little life hack, for doing percentages:

x% of y = y% of x

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

— Ben Stephens (@stephens_ben) March 3, 2019

We keep doing sums to make sure it’s right. Very simple sums, mind. And it appears to be true.

umm so is the answer 3? The suspense is killing me.

— london (@Shehzaadi_kris) March 3, 2019

It is!

18% of 50 feels hard to calculate.

But 50% of 18 is a doddle, right?— Ben Stephens (@stephens_ben) March 3, 2019

And it’s not only our mind that was blown.

As someone who Quite Likes Maths I cannot believe I hadn’t realised this until the age of 47. https://t.co/4MFf8DuB6s

— Rhodri Marsden (@rhodri) March 6, 2019

This is the day Twitter changed my life.

I earn a living as a scientist, juggling numbers every day, and I have never realised this:

X percent of Y is equal to Y percent of X

7% of 50 = 50% of 7 = 3.5

Not sure whether I should be happy or embarrassed… Maybe both. https://t.co/0rpVaJgPUh

— Antoine van Oijen 🔬🧫🧬 (@van__Oijen) March 5, 2019

How did I not know this? 😃 https://t.co/Cf3O8D5H9R

— Hasu (@hasufl) March 5, 2019

God. Damn. https://t.co/JW0mTvhMDF

— Anne Hilborn (@AnneWHilborn) March 5, 2019

More maths sorcery! @SLSingh https://t.co/csU7BbK9PA

— Prof Alice Roberts (@theAliceRoberts) March 5, 2019

And in case you’re one of those people that would like to see the proof, here you go.

Basic arithmetic isn't a "life hack", but OK. Here's a proof. Just rearrange multiplication.

x% of y = (0.01 * x) * y = x * (0.01 * y) = y% of x

The proof reveals another move: if x% of y is hard and y% of x is hard, try multiplying x*y and dividing by 100. E.g., 12% of 12. https://t.co/92uO899DxS

— Jeremy Kun (@jeremyjkun) March 5, 2019

We’re going to level with you here, that just confused us further.

Yes, it's fascinating that it's fascinating. If you say x * y = y * x, it's obvious. Even x/100 * y = y/100 * x seems unremarkable. But we don't think of the % operator as a form of multiplication, so the commutativity comes as a surprise. https://t.co/raSMaVblBq

— Brian Hayes (@bit_player) March 3, 2019

So did that. But not everyone was as impressed as us.

Those of us who studies basic arithmetic at school are scratching our heads as to why this is a revelation ♀️. Next you’ll be telling us that x+y = y+x and that xy = yx

— Ozoda (@OzodaM) March 5, 2019

Cool. Thanks for helping get people enthusiastic about maths. You won't believe this, but a lot of people find mathematicians rather smug and patronising. Crazy, right?

— Ben Stephens (@stephens_ben) March 5, 2019

It’s super basic though. You don’t get people excited about languages by telling them how to spell…

— Ozoda (@OzodaM) March 5, 2019

Sincerely, this tweet and the replies it generated got me excited about math for the first time in decades. I thought “Cool! I get it! And maybe there’s more I can grasp.” Then your comment felt like “No, you’re just a dolt impressed by 1+1. You’ll be lost if you dig deeper.”

— DaveHimslef (@DaveHimslef) March 5, 2019

Some people can't help yucking other people's yum.

— Ben Stephens (@stephens_ben) March 5, 2019