### Traffic Lights

The game uses a 3x3 square board. 2 players take turns to play, either placing a red on an empty square, or changing a red to orange, or orange to green. The player who forms 3 of 1 colour in a line wins.

### Achi

This game for two players comes from Ghana. However, stones that were marked for this game in the third century AD have been found near Hadrian's Wall in Northern England.

### Yih or Luk Tsut K'i or Three Men's Morris

Some puzzles requiring no knowledge of knot theory, just a careful inspection of the patterns. A glimpse of the classification of knots and a little about prime knots, crossing numbers and knot arithmetic.

# Fifteen

##### Stage: 2 and 3 Challenge Level:

Fifteen is a game for two players that you can play anywhere, anytime. Try it without writing anything down. You take it in turns to choose one of the whole numbers 1 to 9 (and each number can only be chosen once). To win you have to pick 3 numbers that add up to 15.

Can you beat the computer?

This text is usually replaced by the Flash movie.

Click on a number to select it.

Can you analyse the structure in the following games and the correspondences between them? Describe the patterns that show they are equivalent games that can be played using equivalent strategies.

We suggest tackling them in the order given.

Printable NRICH Roadshow resource.