A remotely operated vehicle is 12 m below the water surface in a flooded mine shaft. It sends a sonar pulse vertically downwards. The ultrasound has a frequency of 50 kHz50\ \mathrm{kHz}50 kHz and a wavelength of 30 mm in the water. The echo from the bottom of the shaft is detected 0.064 s after the pulse is sent.
How far below the water surface is the bottom of the shaft?
108 m108\ \mathrm{m}108 m
48 m48\ \mathrm{m}48 m
36 m36\ \mathrm{m}36 m
60 m60\ \mathrm{m}60 m