The simple maths trick that could win you a bet next time you’re at the pub

Next time you’re at a BBQ, party or the pub, make a bet that this seemingly unlikely coincidence is true and you’ll likely walk away a winner. 11, 201910:45am

This first grade maths problem has the internet stumped

This first grade maths problem has the internet stumped

Think you're a real maths whiz? A Hong Kong first grade admission test has got people stumped, so why not take it yourself and find out if you're smarter than a first grader?

Cheers to maths.Source:istock

It’s a simple maths question with a seemingly obvious, but incorrect, answer.

And knowing the real solution could be an easy way to win a bet the next time you’re at a party, according to Professor Tim Brown, director of the Australian Mathematical Sciences Institute.

“In a random group of people, how many do there need to be for there to be an evens bet that two of them have the same birthday?” Prof Brown asks.

Given there are 365 days in a year, most people might assume there would need to be something like 365 people in the room, give or take, for that to be the case.

RELATED: Simple maths equation dividing the internet

RELATED: Three-question IQ test most people fail

Counter-intuitively, the answer is actually far fewer — you only need 23 people in the room for there to be a 50 per cent chance two of them share the same birthday.

“Most people think you’d need more than this, and it is a good party game,” he says.

“Get 30 or more people at a party and make a bet that two of them have the same birthday — you are on a winner with a 70 per cent chance that two of them have the same birthday.”

So what’s the reason?

“If we pick two people at random, there is a chance of 364/365 that they will not have the same birthday because no matter what the birthday of the first person is, the second person just has to avoid that birthday,” Prof Brown says.

“With three, the chance no two having the same birthday is 363x364/(365x365) because the third person has two birthdays that they must avoid.”

He adds, “Continuing this way — easy to do in Excel — the chance of no two birthdays the same in a group of 22 people is 52 per cent and with 23 it sneaks down to 49 per cent. So the chance of at least one pair of identical birthdays is 48 per cent in a group of 22 and 51 per cent in a group of 23.”

So next time you’re at the pub, don’t forget your Excel spreadsheet.

Add your comment to this story


Join the conversation, you are commenting as Logout

MORE IN money