A useful math trick, which could have saved hours of our lives has gone viral, leaving a lot of people asking "why the hell wasn't this taught in school?"

A lot of people find percentages difficult, even with the aid of a calculator. For instance, if we told you to calculate 14 percent of 84 in your head, you'd probably close the article and walk away muttering about how much you hate us, so we won't do that.

But, as it turns out there's a really easy way of simplifying percentages, which half of the Internet isn't aware of.

"Maths whizz" Ben Stephens tweeted the trick, and a lot of people are mad they're only just learning about.

The answer is of course 3, which is so much easier to figure out when you're trying to get 75 percent of 4, rather than 4 percent of 75.

Ben tweeted more examples.

"18% of 50 feels hard to calculate," he wrote, and he is right it does on first glance. "But 50% of 18 is a doddle, right?"

Holy crap it's 9!

If you haven't heard of this trick, don't be ashamed, you are not alone. Apart from a few annoyed mathematicians...

Ignoring those, most people thanked Ben for the math hack they had never heard of, including people who work as mathematicians.

"Turns out that pretty much everyone gets that 3 x 5 = 5 x 3," Ben wrote on Twitter. "But that we have never thought (or been taught) to think of calculating a percentage as a multiplication operation."

Of course, the trick doesn't make every percentage easier to calculate.

But for other percentages, when you don't have a calculator to hand, it's a game-changer.