A student stands 60m away from the left wall of a long, empty corridor whose right wall is parallel to the left one. He claps loudly and hears two distinct echoes: the first after 0.35s and the second after 0.85s. If the speed of sound in air is 340m/s, what is the separation between the two walls?
(a) 170m
(b) 205m
(c) 145m
(d) 290m