When an earthquake occurs, two types of sound waves are generated and travel through the earth. The primary, or P, wave has a speed of about 10.5 km/s and the secondary, or S, wave has a speed of about 5.5 km/s. A seismograph, located some distance away, records the arrival of the P wave and then, 140 s later, records the arrival of the S wave. Assuming that the waves travel in a straight line, how far is the seismograph from the earthquake?

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