When the door of the Chapel of the Mausoleum in Hamilton, Scotland, is slammed shut, the last echo heard by someone standing just inside the door reportedly comes 21.0 s later.
(a) If that echo were due to a single reflection off a wall opposite the door, how far from the door would that wall be (in km)?
(b) If, instead, the wall is 27.1 m away, how many reflections (back and forth) correspond to the last echo? Assume the speed of sound is 345 m/s.