Answer & xplanation:
To solve this problem, we need to use the fact that P waves travel faster than S waves and that the time difference between their arrivals at a seismograph station can be used to estimate the distance to the earthquake's focus.
The time difference between the P and S waves in this case is 15 seconds, which means that the earthquake is located somewhere along a circle with a radius of 15 x 8 = 120 kilometers (since P waves travel at an average speed of about 8 kilometers per second).
However, we need to remember that the seismograph is only located within 100 kilometers of the earthquake. Therefore, the correct answer is that the earthquake is approximately 120 - 100 = 20 kilometers away from the seismograph.
So, none of the choices given are correct. The approximate distance from the seismograph to the focus of the earthquake is 20 kilometers.