# It’s a happy birthday to you and you and you!

## It’s a happy birthday to you and you and you! University Challenge breakout star decodes the fascinating MATHS of daily life – including the likelihood of being born on the same day as others

**University Challenge contestant Bobby Seagull discusses maths in daily life****He claims a cook who is incompetent at numbers is an incompetent cook****He shares the probability of having the same birthday as members of a squad**

THE LIFE-CHANGING MAGIC OF NUMBERS

by Bobby Seagull (Virgin £16.99, 288 pp)

Stardom is a strange thing these days. Bobby Seagull did rather well on University Challenge a couple of years ago and, somehow, he has managed to translate this relatively modest achievement into full-blooded celebrity.

But, between appearing on TV and radio — and probably being invited to starry parties — Seagull has another, quieter life: he teaches mathematics at a secondary school and is researching a doctoral thesis on maths anxiety, a fascinating subject.

I, myself, read maths at university a few years ago and, whenever I tell anyone, they squirm with horror and say something like: ‘You must be terribly clever.’ Even people who are themselves terribly clever tell me this.

University Challenge contestant Bobby Seagull, reveals the fascinating ways maths informs everyday life including the likelihood of sharing birthdays in a new book (file image)

It’s the abstraction that worries a lot of people (what exactly are X and Y?), but Seagull operates in the real world and illustrates his examples with the ease of the born educator.

Here’s one. In a discussion about prime numbers — all the numbers greater than one that can’t be divided into smaller numbers: two, three, five, seven, 11, 13, 17 and so on — he tells us of a species of cicada that leaves its burrow in intervals of seven, 13 or 17 years.

It has developed these natural rhythms through evolution, using prime number years to minimise its chances of overlapping with predators.

So, if a predator had a five-year life cycle and the cicadas emerged every 15 years, they’d be devastated every time. But with a 17-year cycle, the cicadas and the predators coincide only every 85 years. I find this truly mind-blowing.

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Seagull writes entertainingly about how maths informs almost all of daily life. ‘Show me a cook who is incompetent at numbers and I will show you someone who is an incompetent cook.’

It’s all about patterns. Seagull was working at Lehman Brothers as a trader in the run- up to the 2008 crash. He knew that something was wrong because the stationery cupboards stopped being replenished.

He’s good on probability, too. In Gareth Southgate’s England squad at the recent World Cup, two of the 23 players shared the same birthday. What are the chances of that? Well, slightly more than half, as it happens.

THE LIFE-CHANGING MAGIC OF NUMBERS by Bobby Seagull (Virgin £16.99, 288 pp)

Take 23 people at random and work out the probability that they all have different birthdays. The probability that the first person has a unique birthday is 100 per cent.

For the second, one day is gone, but the other 364 are free. So his probability is 364/365. The next one is 363/365, and so on, until you reach the 23rd, whose probability of having a unique birthday is 343/365.

The probability that they all have unique birthdays is these 23 probabilities multiplied together, which comes to slightly less than half — 49.1 per cent.

So the probability that (at least) two of your 23 actually share a birthday is 50.9 per cent.

And, if you now have a splitting headache, I can’t say I’m hugely surprised.

Clever, though, don’t you think? As is this thoroughly likeable little book.

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