A specimen consists of two large metal plates “A” and “B”, which are joined together one on top of the other with no air gap between the plates. Each plate is 10mm thick. Assume plate “A” is on top. An ultrasonic instrument is correctly calibrated for 0 to 100 mm testing range for testing plate “A” using a 45º shear wave probe. This probe is placed on top of plate “A” and a signal is found to appear from a defect at 20 mm on the screen. This defect is in plate “B”. What is distance of this defect from the top of plate “A”?

      The velocity of ultrasound in plate “A” is 3,000m/s

      The velocity of ultrasound in plate “B” is 2,500m/s