The diagram shows waves of sound travelling through the air. by which factor is the sound intensity decreased at 3 meters?

The diagram is missing; however, we know that the intensity of a sound wave is inversely proportional to the square of the distance from the source:

where I is the intensity and r is the distance from the source.

We can assume for instance that the initial distance from the source is r=1 m, so that we put 

The intensity at r=3 m will be

Therefore, the sound intensity has decreased by a factor .