Nah it's a tough subject. I screwed it up too when I first looked at it, and I have a four year degree in it.

There is a more intuitive way of looking at it:

At the time when the P wave reaches the station, it has gained 19s * 3.6 km/s = 68.4km on the S wave. The P wave moves at 7km/s and opens up a gap of 3.4 km/s. So the ratio of distance moved/gap opened up is 7/3.4 = 2.058km per km in gap. Therefore the total distance it has travelled is 64.8*2.058 = 140.8km.