15 was a number.
Susan Foreman was fifteen years old when Ian Chesterton and Barbara Wright first met her grandfather. (TV: An Unearthly Child)
The Archangel Network was composed of fifteen satellites orbiting the Earth. (TV: The Sound of Drums/Last of the Time Lords)
When the Cybermen attempting to capture the moonbase first detected the relief ship from Earth, their computer predicted it would land on the lunar surface in fifteen minutes. (TV: The Moonbase)
The interior of the Doctor's TARDIS decreased in size by fifteen centimetres following the Doctor's regeneration into his second incarnation. (PROSE: Invasion of the Cat-People)
Jim Hardwicke spent fifteen years trying to develop an anti-aging drug, during which he endured much ridicule from his colleagues. (PROSE: The Living Death)
On Red Rocket Rising, Tom Cardwell's bio-scan indicated that the Eighth Doctor's body temperature was fifteen degrees, which the Doctor attributed to the cold. (AUDIO: Blood of the Daleks)
Of the 27 different species that the Twelfth Doctor identified as living in Trap Street, 15 were known for aggression. (TV: Face the Raven)
When Rose Tyler was fifteen years old, she was suspended from Jericho Street Comprehensive for three days after she convinced the choir to go on strike. (PROSE: Meet Rose)
23 November (TV: Death in Heaven) 2001 marked Clara Oswald's fifteenth birthday. Less than a month earlier, she complained to her mother that she never let her do anything despite being "nearly fifteen". (PROSE: Clara Oswald and the Enchanted Forest)