Seismic Waves Analogy

We know how big the hot areas inside the mantle are because we can calculate it from the time it takes earthquake vibrations to travel through the planet. Here is an analogy to show how the math is done.

1. Your friends live 100 miles away.
2. Assume they can travel at either 50 mph or 25 mph depending on traffic (a simplification).
3. They call you to say they are leaving to come see you.
4. Calculate how long it will take them to reach you if there is no traffic: 100 miles / 50 miles each hour = 2 hours travel time.
5. How long will it take if they hit traffic the whole way? 100 miles / 25 mph = 4 hours travel time.
6. This means that 100 miles of traffic will cause them to arrive 2 hours later than if they did not hit traffic.
7. They arrive after travelling for 3 hours
8. The slow traffic made their travel time 1 hour longer than if there was no traffic.
9. How delayed were they as a percentage, compared to the 2 hour delay that would have happened with 100 miles of traffic?  1 hour delay / 2 hour delay = 1/2 or 0.5 = 50%
10. Since their delay was 50% of  the delay caused by 100 miles of slow traffic, we know that   50% of the 100 miles was slow.
11. 50% of 100 miles = 50 miles of slow traffic.

Obviously calculating using seismic waves in a lot more complex, but this is basically how its done.