Problem 4 [20 points]: A 20in thick concrete slab is inspected using the impact echo method. The Fast Fourier Transform (FFT) obtained at a test location is given here: Relative Amplitude 6kHz Frequency (kHz) The speed of the longitudinal wave in the region of concrete tested is 10,000 feet/sec. Please assess the condition of the deck at such location by leveraging calculations and provide a clear explanation.