Number 7


7 is a prime number.

1, 3, 4, 7, 11, 18, 29...
A Lucas number.

= 23 - 1
A Mersenne prime.

There are seven days in a week.

Hept- or Sept- means seven. A heptagon is a figure with seven sides and a heptachord is a seven-stringed musical instrument. A septennium is a period of seven years and September used to be the seventh month in the year, but not any longer.

The Seven Deadly Sins are avarice, envy, gluttony, lust, pride, sloth and wrath (listed in alphabetical order, not order of wickedness).

Among many things that come in sevens are the Seven Wonders of the Ancient World, the Seven Sisters, Shakespeare's Seven Ages of Man, the Seven Levels of Hell, and the Seven Dwarves.

Netball and water polo are both played with teams of seven players.

In Britain the 20p and 50p coins both have seven sides.

Under British law, when you reach the age of seven you can open and draw money from a National Savings Bank account or a Trustee Savings Bank account.

7-Up is a soft drink. It was invented in America in the 1920s by Mr C L Griggs of Missouri who originally called it Bib-label Lithiate Lemon-Lime Soda. With a name like that sales were poor even though the drink tasted good and so Mr Griggs set about changing the name. After six attempts he came up with 7-Up, or so the story goes. 7-Up is also the name of a card game.

John Sturges's 1960 western The Magnificent Seven is about a Mexican village that hires seven gunmen for protection from bandits. The story is based on an earlier Japanese film made in 1954 - Akira Kurosawa's The Seven Samurai.

Roy Sullivan, a park ranger from Virginia, USA is the only person to have been struck by lightning seven times. Between 1942 and 1977 he was struck on top of his head (twice), his eyebrows, his shoulder, his chest, his ankle and his big toe. Although he received hospital treatment for his injuries, he was extraordinarily lucky to escape death from so many strikes.

A version of this familiar problem appears on the Rhind papyrus written in Egypt about 1650 BC –

As I was going to St Ives,
I met a man with seven wives,
Each wife had seven sacks,
Each sack had seven cats,
Each cat had seven kits:
Kits, cats, sacks and wives,
How many were going to St Ives?

How many were going to St Ives?

A wheel with seven spokes
A wheel with seven spokes.

Ask a number of different people to give you any number between one and ten, and most will choose seven. Ask people to name their favourite number between one and ten, and again most will say seven.

In 1956 George Miller wrote an article The Magical Number Seven Plus or Minus Two: Some Limits on Our Capacity for Processing Information. This showed that the amount of information which people can process and remember is often limited to about seven items. One example of this is called the digit span.

Ask someone to repeat back to you exactly what you say. Begin with four digits chosen at random e.g. 6 6 2 5. Then give them five digits e.g. 5 8 4 5 0, then six, and so on. Carry on increasing the number of digits until they make a mistake. The longest number of digits they get completely right is called their digit span and for most people this is about seven digits.

Suppose someone is shown a pattern of dots for a very short time - just one fifth of a second - and they are asked to count the number of dots they saw. If the number is less than seven they will be right almost every time, but with more than seven, they will make lots of mistakes.

Seven is not really a magic number, but does have an uncanny way of appearing in all sorts of odd situations.

Four linked hexagons

There are seven different ways of linking four hexagons together. Here is one of them. Can you find the other six?

Thomas Vella-Zarb, a retired pharmacist and high school teacher from Ontario, tells this story –

'As a kid, my mother's favourite was the question about a farmer selling eggs. The first customer asked for “Half of what you have in your basket plus half an egg”. The farmer obliged without breaking any eggs. The second customer likewise asked for “half of what you have plus half an egg”. Once again he obliged. The third customer asked for the same amount, “Half of what you have plus half an egg.” The farmer said, “Then you take all that I have in my basket.” Of course my mum would ask us to figure out how many eggs he had to start with.'

You can probably guess the answer to this but you might like to check you are right. Then try to figure out how many eggs if there are four customers rather than three in the story? What happens with five customers? ... with ten customers?

Nautical 7 km/h sign