physics
posted by Kate .
Earthquakes are essentially sound waves—called seismic waves—traveling through the earth. Because the earth is solid, it can support both longitudinal and transverse seismic waves. The speed of longitudinal waves, called P waves, is 7800m/s . Transverse waves, called S waves, travel at a slower 4400m/s . A seismograph records the two waves from a distant earthquake.
If the S wave arrives 2.1min after the P wave, how far away was the earthquake? You can assume that the waves travel in straight lines, although actual seismic waves follow more complex routes.

dp=v*t=7800*t
ds=4400(t+2.1)
but dp=ds
set the equations equal, solve for t.
then put it in either equation, and solve for the distance.