A swimming pool is 10 m wide. A loud sound is made in the water 2.0 m from one wall and the reflected sounds are detected with a microphone placed next to the sound source. The reflection from the wall 8.0 m away arrives 0.010 s after the reflection from the wall 2.0 m away. What is the speed of sound in water?

Answer:

19) E

It is important to understand the situation.

The last line states that the reflected sound from the 8m wall reflects from the 2m wall and is then detected by the microphone.
So the waves cover the entire width of 10m and the extra 2m.

So total distance covered by sound waves = 10 + 2 = 12m
Time = 0.010s

Speed = Distance ÷ Time = 12 ÷ 0.01 = 1200 m/s