Fatto generale: 15540 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Fatto matematico: 15540 is an uninteresting number.
Trivia: 15540 is an unremarkable number.
Data storica: July 18th is the day in 1857 that Louis Faidherbe, French governor of Senegal, arrives to relieve French forces at Kayes, effectively ending El Hajj Umar Tall's war against the French.