A bat, flying toward the east at 2.0 m/s, emits a shriek that is reflected back to it from a wall that is 20.0 m in front of the bat at the instant the shriek is emitted. Sound travels at 340 m/s in the air. How many milliseconds after emitting the shriek does the bat hear the reflected echo from the wall? *

t = S / V     where S is distance sound travels and V the speed of sound

S = 20 + 20 - v t      where v is speed of bat and t time to hear echo

S = 40 - v t

t = (40 - S) / v = (40 - V t) / v      substituting S from 1st equation

v t + V t = 40

t = 40 / (v + V) = 40 / (340 + 2) = .117 sec = 117 ms

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