An echo-sounder in a traveler (fishing boat) receives an echo from a shoal of fish 0.4s after it was sent. If the speed of sound in water is 1500 m/s, how deep is the shoal?

Given:
Speed of sound in water: 1500 m/s
Total Time: 0.4s
Deep(distance travelled)? 
Speed = distance travelled/time taken
1500 = distance travelled / 0.4
1500 x 0.4 =  distance travelled 
600 = distance travelled
Distance between boat and shoal of fish is 600/2 = 300m