Alistener triples his distance from a source that emits sound uniformly in all directions. by how many decibels does the sound intensity level change?

Sound intensity goes inversely as the square of the distance from the source. This is easy to understand if you think of the total sound energy spread out over the surface of a sphere with the source at the center. The surface area of the sphere goes as r^2, so the power density goes inversely as such.

The decibel change is simply

Change = 10 log10( (1/3)^2 ) = -9.54 dB

0.275 m/s in the opposite direction

Explanation:

m = Mass of student = 50 kg

M = Mass of plank = 150 kg

= Velocity of the student with respect to the plank = 1.1i m/s

As the linear momentum is conserved in the system

The plank will move at 0.275 m/s in the opposite direction