An ultrasound pulse travels through a 20 mm layer of tissue P, then through 30 mm of tissue Q. It reflects at the far surface of Q and returns along the same path.
The speed is 1000 m/s in P and 1500 m/s in Q.
What is the total time between sending the pulse and receiving this echo?
80 μs80\ \mu\mathrm{s}80 μs
40 μs40\ \mu\mathrm{s}40 μs
100 μs100\ \mu\mathrm{s}100 μs
160 μs160\ \mu\mathrm{s}160 μs