Figuring out percentages can be a nightmare - especially if you haven't got a calculator to hand.
But now, a maths whizz has blown people's minds on Twitter after sharing a simple way of working out complicated sums.
The "fascinating little hack" involves flipping the numbers around to make things easier.
For example, if you need to work out 18% of 50, swap them and go for the easier option of 50% of 18 instead. The answer for both is nine.
Another example is 4% of 25. Flip it round to work out 25% of 4 - and the answer to both is one.
The formula was tweeted by Ben Stephens from Brighton, and it has since attracted more than 11,600 likes and 4,000 retweets.
Some of his followers have described the trick as "life changing", with one writing: "Not sure whether I should be happy or embarrassed. Maybe both."
Others said they were "terribly impressed" and questioned "why was I never taught this in school."
Mabel Marie said she "loved how many minds this is blowing. Hooray for stimulating more maths conversation in the world! Woohoo!"
After claiming to spend years on Twitter "banging on about Trump and Brexit" - Mr Stephens seemed to find the huge response to his trick quite amusing.
He said: "I pop out one tweet about maths (a subject about which I have never tweeted before) and it sets the internet on fire."
© Sky News 2019