A young lady was sitting alone at the bar in the Jolly Quizmaster, and attracting quite a bit of attention from the other customers. She was extremely well dressed, in what looked like designer label clothes. And we had not had anyone drinking pink gin for a long, long time - who knows how long the Angostura bitters had been behind the bar!

Presently she took the very latest iPhone out of her designer handbag and busied herself with whatever people of her age do on their mobile phones.

Talking of her age, a lot of the groups in the bar were doing exactly that, speculating on her exact vintage. Someone must have spoken a little too loudly, as she put down her phone and looked him in the eye.

"If you really want to know my age, you will have to work it out. My mother's age is half the sum of my father's age and my age. My father's and mother's ages when added together make 100. Both my father's age and my mother's age are prime numbers."

With this she resumed attending to her phone, and studiously ignored the room. Of course, behind her back, pens and paper were employed on the calculation.

So how old was she?

There are three pairs of prime numbers that add up to 100, namely 29 and 71, 41 and 59, and 47 and 53.

Clearly the first pair are not the answer as 29 (the mother's age) is less than half of 71 (the father's age).

With the last pair, twice 47 (the mother's age) is 94, which is 53 (the father's age) plus 41 (the lady's age). This is not feasible as the mother would have had to give birth at the age of six.

With the middle pair, twice 41 (the mother's age) is 82, which is 59 (the father's age) plus 23 (the lady's age). This is does work.

The young lady in the bar was 23.