A pulse travels through 12 m of water at 1500 m/s and then through 18 m of sediment at 3000 m/s. It reflects and returns along the same path. What is the total travel time?
28 ms28\ \mathrm{ms}28 ms
20 ms20\ \mathrm{ms}20 ms
40 ms40\ \mathrm{ms}40 ms
14 ms14\ \mathrm{ms}14 ms