The number 15 is the third number in the numbers sequence. Below is a list of the appearances of this number in chronological order by episode, and the number 15 in real life.

In Lost:

Season 1:

"Tabula Rasa"-

• Ray Mullen said the nearest town is 15 kilimeters away.

"House of the Rising Sun"-

• The decorator told Sun to leave the airport at 11:15.

"Hearts and Minds"-

• Boone flew for 15 hours to save Shannon.

"Outlaws"-

• The whisky Sawyer and Christian Shephard drank had 15 on it.

"Born to Run"-

• Kate and Tom Brennan buried the time capsule 15 years ago.

"Exodus: Part 1"-

• Shannon says she doesn't want to be sitting next to a crying baby for 15 hours.

"Exodus: Part 2"-

• The raft was 15 miles offshore when the blip on the radar was discovered.

Season 2:

"Collision"-

• Teresa Cortez's office door is 315.

"Lockdown"-

• Anthony Cooper's safety deposit box number was 1516.

"S.O.S."-

• 15 people started to help with Bernard's SOS sign.

"Two for the Road"-

• One of the police cars had 15 on it.

Season 3:

"A Tale of Two Cities"-

• The time on Jack's pager was 7:15:23.

"The Glass Ballerina"-

• The hotel room Jae Lee was at was 1516.

"The Cost of Living"-

• Mr. Eko's stick had the number 15 on it.

"Flashes Before Your Eyes"-

• The Playpen Magazine in Sawyer's stash was issue 15.
• The photo cart had the number 15 on it.

Season 4:

"Confirmed Dead"-

• In the room Miles goes to to exorcised the spirit, there is a poster with the number 15 on it.

"There's No Place Like Home: Part 1"-

• The Dharma Initiative cracker Hurley eats is 15 years old.

In Real Life:

• There are 15 days in a month in the Chinese calender.
• 15 is the number of the devil in the Tarot Cards.
• Passover begins on the 15 day of the Hebrew month of Nisan.