You are on a boat in a fog and know there are cliffs ahead of you somewhere, but you cannot see them. You use your fog horn to send out a blast of sound (assume the temperature of the air is 20 ^oC) and the time it takes for the sound to return to you is 20.0 s. How far away are the cliffs (in metric units)? Hint: Be careful and think about the physics of the problem. You may want to make a sketch of the situation to help you determine the answer.