An aluminium plate is inspected with a single ultrasound transducer placed on its top surface. Two distinct echoes are observed: the first returns after 40μs and the second (from the back surface) after 60μs. If the speed of sound in aluminium is 6000m/s, how far below the top surface is the internal flaw that produced the first echo?
(a) 18cm
(b) 12cm
(c) 6cm
(d) 24cm