# 20

= 2 × 2 × 5

= 2 × 10

= 4 × 5

= 2 + 4 + 6 + 8

= 1 + 3 + 6 + 10

A tetrahedral number - the sum of the first four triangular numbers.

A score is another name for 20. Counting in twenties is common in many languages and cultures. 'Three score years and ten' means 70 years. Marks are sometimes given 'out of 20' and popularity is measured by a 'top 20'.

A china anniversary celebrates 20 years.

In old British coinage 20 shillings made one pound. In old French coinage 20 sous made one franc.

A darts board is divided into 20 sectors numbered from 1 to 20.

There are 20 hundredweight in one ton.

An icosahedron is a solid with an equilateral triangle on each of its 20 faces. It is not possible to make a regular solid with more than 20 faces.

*An icosahedron makes a 20 sided die.*

A dodecahedron is a solid with 20 corners (or vertices).

Twenty-twenty vision is perfect eyesight, proved by reading letters on a chart from 20 feet.

* Twenty* is the name
of a village in Lincolnshire near Bourne.

Human infants normally grow 20 teeth, which are replaced by 32 adult teeth as they grow up.

In Roald Dahl's *Charlie
and the Great Glass Elevator *one pill of Wonka-Vite has the effect
of making you exactly 20 years younger.

** This is a puzzle where
you have to arrange the numbers from 1 to 20 in the correct order. The
numbers slide around the track and the wheel lets you rotate any four
of the numbers.**

** Would the puzzle be easier
or harder if the wheel let you rotate just two numbers? What would happen
if you could rotate three numbers?**

A *hussar* is a soldier
on horseback. The word comes from *husz*, the Hungarian for 20. This
was the number of households which had to be taxed to provide for one
soldier. In the sixteenth century the Hungarian King Mathias Corvinus
ruled that each group of 20 households in a town was to provide one soldier
on horseback fully equipped.

*20 Questions* is a game
where objects have to be identified by asking up to 20 questions which
can only have the answer 'yes' or 'no'.

** Have you played the numbers
version of 20 Questions? One person secretly writes down a number
and the others ask questions like 'Is it an even number?', 'Is it more
than a thousand?', 'Does it have a 6 in it?'. **

** The game gets harder if
you use the rule that each question must be of a different type.**