A girl standing 150m in front of a building fires a shot using a starting pistol. A boy standing 350m behind her hers two bangs 1s apart. From this information, what is the speed of sound in air?
(a) 300m/s
(b) 330m/s
(c) 500m/s
(d) 350m/s
Please explain how you got the answer!

Question

A girl standing 150m in front of a building fires a shot using a starting pistol. A boy standing 350m behind her hers two bangs 1s apart. From this information, what is the speed of sound in air? 
(a) 300m/s 
(b) 330m/s 
(c) 500m/s 
(d) 350m/s 
Please explain how you got the answer!

mani_jha · Answer

Well, the boy is 350 m apart from the source of sound. So, sound is taking 1s to reach him. Now, what is the speed?

anonymous · Answer

just a general doubt: why does the boy hear two bangs?

mani_jha · Answer

Yes, the question should be" A boy sees the girl fire and hears a bang 1 second later"

anonymous · Answer

I think the boy hears two bangs because of the echo.

jamesj · Answer

Yes, one is the direct sounds he hears, then next he hears the echo.

The first sound reaches him after traveling 200 m.
The second after traveling 150*2 + 200 = 500 m (to the building, echo, back from the building to the girl firing the gun, then beyond that person to the boy)

Therefore the two sounds have a distance difference of 300 meters.  Hence if those two sounds are one second apart, the speed of sound is ...

jamesj · Answer

Make sense?

anonymous · Answer

The speed of sound would be 300m/s. Thank you so much! :) You made it so much easier to understand.

anonymous · Answer

JamesJ: "Therefore the two sounds have a distance DIFFERENCE of 300 meters." Precisely- That's all that matters, here. Often in tests it's fastest to not work it all out.

anonymous · Answer

i.e. it doesn't matter how far away from the girl the boy is, as long as he's further from the building than the girl. 2*150 m and 1 s are the only figures you need.