Merriam-Webster's Advanced Learner's Dictionary:
fifteen
fif·teen /ˌfɪfˈtiːn/ noun, pl -teens [count] : the number 15
- fifteen adj
• fifteen dollars
- fifteen pronoun
• Only fifteen (of them) showed up on time.
- fif·teenth /ˌfɪfˈtiːnɵ/ noun, pl -teenths [count]
• the fifteenth of September
• one fifteenth of the total
- fifteenth adj
• the fifteenth day
- fifteenth adv
• Their boat finished fifteenth.