(a) What is the ratio of the intensity of an earthquake wave as it passes a point 20 km from the source to the intensity as it passes a point 15 km from the source?

(b) What is the ratio of the amplitude of an earthquake wave as it passes a point 20 km from the source to the amplitude as it passes a point 15 km from the source?

Intensity of earth quake is based on the observed effects of ground shaking on people,
buildings, and natural features. It varies from place to place within
the disturbed region depending on the location of the observer with
respect to the earthquake epicenter while magnitude is related to the
amount of seismic energy released at the hypocenter of the earthquake.

As the distance from the source increases the intensity decrases hence they are inversely porportional