Sound from a 55dB horn is measured at a distance. What is the intensity of the sound in W/m2?

Answer 1

1 W/m² is defined as 120 dB.
For every 10 dB higher, power is multiplied by 10 times.
Inversely, for every 10 db lower, power is reduced (divided) by 10 times.
For example,
100 db = (120-20) dB=1W/m²*10^-2
75 dB = (120-45) dB = 1W/m²*10^(-4.5)=10^(-4.5) W/m²

You can work out 55dB in W/m² similarly.

Also, read the following link for more information.
http://www.physicsclassroom.com/class/sound/Lesson-2/Intensity-and-the-Decibel-Scale