Numbers round
One contestant selects six of twenty-four shuffled tiles. The tiles are arranged into two groups: four "large numbers" (25, 50, 75 and 100) and the remainder "small numbers", which comprise two each of the numbers 1 to 10. The contestant dictates how many large numbers are in the selection; anywhere from none to all four. A random three-digit target is generated by an electronic machine, affectionately known as "CECIL" (which stands for Countdown Electronic Calculator In Leeds).[37] The contestants then have thirty seconds to get as near to the target as possible by combining the six numbers selected with addition, subtraction, multiplication and division.[32] Numbers can be used as many times as they appear in the selection, and need not all be used.
Decimals and fractions are not allowed - only integers may be used at any stage of the calculation.[32]
Points are awarded for the closest solution, and again both contestants score if the solutions are equally close. 10 points are given for an exact answer, 7 points for a non-exact solution up to 5 from the target, and 5 points for a solution between 6 and 10 from the target. If neither contestant can get within 10, no points are awarded.
Source:
http://en.wikipedia.org/wiki/Countdown_(game_show)#Numbers_round