## Dating somebody with the same birthday

In a group of people, there's per cent probability that two people will have the same birthday because there are only days in a year, excluding leap year. This mathematical head-scratcher, called the birthday paradox, says that you only need to take a random sample of 23 people to have a 50 per cent of two of them sharing the same birthday. While the sample size seems impossibly small, the combined comparisons quickly rack up.

The first person compares their birthday with 22 others, but the second person compares with 21 as they have already compared with the first person. Each subsequent person compares with one less. In total, it means that just 23 people can provide chances to match birthdays, giving a chance there will be a match. Conversely, with just two people the chances of them sharing a birthday is With three people, the odds decrease just barely to But at 15 people, it's down to Among 23 people there are potential pairings, says an article in the Washington Post.

By the time you reach 60 people, it's odds on you will find a birthday match. One famously serendipitous story occurred in when American children's book author Anne Parrish was browsing the used bookshops along the Seine in Paris when she came across a copy of one of her favourite books, 'Jack Frost and Other Stories,' by Helen Wood.

She bought the book and showed it to her husband, who handed it back a few moments later, pointing out the inscription inside, which read: As incredible as this coincidence sounds, Mazur put the odds at 3, to 1 - or 'slightly better than the odds of being dealt a poker hand of four of a kind.

The famous illustration of probability is the infinite monkeys theorum, which ties together random events to make something meaningful by chance. In states that an infinite number of monkeys writing for an infinite amount of time could produce the entire works of Shakespeare Mazur also investigates how the actor Anthony Hopkins would find a copy of the author's own book - marked in his hand - of the film he was working on at the time, 'The Girl From Petrovka' by George Feifer, on a bench in the London Underground in The answer, says Mazur, is found in the rational order of the universe.

For some other random tests, the probability of achieving the unlikely can take much longer. In , mathematician Emile Borel raised the issue of whether a number of random events could amount to something meaningful. That question subsequently morphed into, 'Could a monkey randomly hitting the keys of a keyboard type out a Shakespearean sonnet? He says the probability of a monkey randomly typing the word 'shall' - as in 'Shall I compare thee to a summer's day' is nearly 1 in 12 million.

The more the monkey types, the better its chances.