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Wave Speed: Earthquakes

WAVES-2X9YLN

An earthquake gives off two types of waves, called P and S waves. Typically P waves are faster, averaging $v_P = 8.5 km/s$, while S waves are slower, averaging $v_S = 5.5 km/s$. A geologist studying earthquakes detects a P wave 40 seconds before he detects the S wave.

How far away was the epicenter (the origin of both waves) from the geologist?