Scientists have developed fairly detailed models of Earth's interior (figure 1A above). Those models are based on principles, checked in laboratory experiments, that: (1) the speed of seismic waves depends on the temperature and density of the material through which they pass, and (2) seismic waves change their direction of motion when they run into boundaries between zones of different density (right-hand figure above). One type of seismic wave, the S- ("shear") waves, cannot pass through liquid. Your local seismograph station detects an earthquake. The P-waves, which travel about 6 km/sec (4 miles/sec) near Earth's surface, arrive 300 seconds before the S-waves, which travel about 4 km/sec (2.5 miles/sec). How far away is the earthquake