= 3 × 7
= 1 + 2 + 3 + 4 + 5 + 6
A triangular number.
1, 1, 2, 3, 5, 8, 13, 21...
A Fibonacci number.
The total number of spots on a normal die.
Formerly the legal coming-of-age when you received the 'key of the door'.
A game of table tennis is won by the first player reaching 21 points.
We live in the twenty-first century.
A twenty-one gun salute is fired in the UK for royalty and in the US for the President. It comes from the time when the largest ships of the British navy had 21 guns along one side.
Twenty-one is a card game which is also known as Pontoon or Vingt-et-un. Its object is to have a total score in one's hand nearest to, but not exceeding, 21.
An unusual grating with 21-fold symmetry.
Under British law, when you reach the age of 21 –
- you can become an MP,
- you can hold a licence to drive a large passenger vehicle or heavy goods vehicle,
- you can apply for a licence to sell alcohol.
21 is the third star number. Star numbers can be represented by a square with a triangle on each side. Here a 3 × 3 square is surrounded by four triangles.
What is the first star number that is greater than a hundred?