Basically, the aim is to reach a target from 100 to 999 by combining six numbers using addition, subtraction, multiplication or division. The "Generate" button will select these six numbers and the target, though they can alternatively be typed in. The "Calculate" button will attempt to solve the problem. The Java code can be seen here; be warned that it is not very object oriented.
The game is derived from the television programme Countdown, which came from the French TV gameshow les chiffres et les lettres. A fuller description and another java version can be found by visiting the Channel 4 website and drilling down through "entertainment", "countdown" and "play the numbers game". Or you could try going straight to the applet.
The Java applet at the top has been copied (with permission) as part of Saxon Publishers' internet excursions. There is not a return link back because they were worried about some of the language in part of the Statistics Jokes collection.