The arithmetician is the member of the presenting team who is charged with providing solutions to the numbers rounds if such a solution evades the contestants. For most of Countdown history, the role was occupied exclusively by Carol Vorderman, who gradually expanded her role to include putting up the numbers and letters and generally acting as the co-presenter. When Vorderman was replaced by Rachel Riley in 2009, Riley was billed as the new arithmetician but, with the exception of the guest introductions, adopted the full co-presenting role.
Dr Linda Barrett was also a Countdown arithmetician (or "vital statistician", as the credits then termed it) during Series 1 and 2. Susie Dent appeared as arithmetician in the 1997 Christmas special, and Joe Wilkinson became the show's first male arithmetician, when he filled in for Riley in Episode Cats2.