A fisherman uses sonar to find a shoal of fish. A pulse of ultrasound is sent out and the reflection is detected 0.4 seconds later.

How long did it take the sound to travel from the boat to the fish?

The sound left the boat, went to the fish, bounced off of the fish, went back to the boat, and was detected after 0.4 seconds.

The sound took 0.2 seconds to go from the boat to the fish, and another 0.2 seconds to go from the fish back to the boat.