Two tall, parallel building faces stand 120m apart along a quiet, narrow lane. A student stands exactly midway between them and bursts a small fire-cracker.
(a) Calculate the time interval between the first and the second echoes heard by the student. (Take the speed of sound in air as 340m/s.)
(b) The student then walks 20m toward one of the buildings and bursts another cracker at the new position. Determine, in the order they are heard, the times (measured from the instant of the burst) at which the student receives the first three echoes. Show all steps in your working.