20 (number)

20 was a number.

As a defence mechanism for the Doctor's TARDIS there were twenty wrong holes within the lock and one right one. Attempting to open the door through a wrong one would cause the lock to melt, preventing access entirely. (TV: The Daleks)

The Tenth Doctor made the observation to Richard Lazarus that some people lived more in twenty years than others did in eighty. (TV: The Lazarus Experiment)

The fruit flies of Hoppledom Six had a lifespan of only twenty minutes. (TV: The Pandorica Opens)

Vivien Rook met Lucy Saxon at 10 Downing Street, ostensibly to request a 20 minute interview for a front page profile piece on "Britain's First Lady" and "The Power Behind the Throne". In reality, however, this was a ruse for Vivien to bring her findings on Harold Saxon to his wife. Lucy, fully aware that her husband was, accepted a 20 minute interview only to leave Vivien to be killed by the Toclafane. (TV: The Sound of Drums)

Room 12, a room within the Twelfth Doctor's confession dial, contained a portal to Gallifrey that was behind a 20 foot thick azbantium wall. (TV: Heaven Sent)

Behind the scenes
According to The Last Dalek, a video game based on the events of the television episode Dalek, Adam Mitchell was 20 years old in 2012.