![](https://cos-cdn.shuashuati.com/pipixue-web/2020-1231-2005-12/ti_inject-812ce.png)
Echo-sounder is usually installed at the bottom of observation ship. The water line of ship is 15m. The sound speed is meanly 1500m/s. The time interval between receiving and launching sound wave is 3s for echo-sounder. How many is the water depth? ( )