Earthquakes are motions of the earth's crust. Essentially, they are big sound waves that travel through the earth. There are two types of waves. The first type is called a longitudinal or P wave, where the earth compresses in the same direction as the wave travels. The second type is called a transverse, or S wave, where the compression in the earth is perpendicular to the direction of the wave. P waves travel roughly twice as fast as S waves.

A seismograph is a device for measuring these waves. By measuring the time between the arrival of each wave, we can calculate the distance to an earthquake. A seismograph measures a P wave and then an S wave 30 seconds later. If the P waves travel at 6000 m/s, how far away was the earthquake **in kilometers**?

**Details and assumptions**

- Assume the waves travel in straight lines.

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