Answer:
1 km
Explanation:
The round-trip time through layer 1 is given as 2 seconds. The total of round-trip times through layers 1 and 2 is 2.5 seconds, so the round-trip time through layer 2 is ...
2.5 s - 2.0 s = 0.5 s
Then the one-way time through layer 2 is 0.5/2 = 0.25 s. The distance traveled by the wave in that time is ...
distance = speed × time = (4 km/s)×(0.25 s) = 1.0 km
Layer 2 is 1 km thick.