There are 60 minutes in an hour.
In ¾ of an hour there are (60 × ¾) minutes = 45 minutes.
In ¾ of an hour there are (60 × 45) seconds = 2700 seconds.
Siren sounds for every 5 seconds.
In 2700 seconds = 2700/5 = 540 times.
The count start after the first sound, the Siren will sound 541 times in ¾ of an hour.