A precast concrete panel is 18 cm thick. An ultrasonic pulse sent into one face returns after reflection from the opposite face in 0.080 ms. What is the speed of sound in the concrete?
4.5×105 m/s4.5\times10^5\ \mathrm{m/s}4.5×105 m/s
4.5×106 m/s4.5\times10^6\ \mathrm{m/s}4.5×106 m/s
2.3×103 m/s2.3\times10^3\ \mathrm{m/s}2.3×103 m/s
4.5×103 m/s4.5\times10^3\ \mathrm{m/s}4.5×103 m/s