Science Homework Help

The Question: A boat is floating at rest in dense fog near a large cliff. The captain sounds a horn at water level and the sound travels through the salt water (1470 m/s) and the air (340 m/s) simultaneously. The echo in the water takes 0.40 s to return. How much additional time will it take the echo in the air to return. 

Relevant equations: v = distance/time 

 Please do not copy/paste answers from other sites such as yahoo or physics forum as I have already used them before I came here.