Fait général: 7785 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Fait mathématique: 7785 is a value of n for which 5n and 6n together use each digit exactly once.
Culture générale: 7785 is an unremarkable number.
Date historique: April 24th is the day in 1918 that First tank-to-tank combat, at Villers-Bretonneux, France, when three British Mark IVs met three German A7Vs.