Sound takes 5 seconds to go one mile. Clark is standing near a rock wall and when he shouts, it take

Sound takes 5 seconds to go one mile. Clark is standing near a rock wall and when he shouts, it takes 20 seconds for the echo to reach his ears. How far away is the rock wall?

The sound makes a round trip from Clark to the wall back to Clark.

20 seconds / 5 seconds per mile = 4 miles

4 miles / 2 for round trip = 2 miles