A sonographer presses a piezo$-$electric transducer against your skin. It operates at a frequency of $3\mathrm{.}6$ MHz$.$ What is the measured time delay, per centimetre of depth of tissue, between the emitted and received signals? Give your answer in microseconds; don't enter units. Reminder: Your skin, much like salty water, allows sound to travel at $1540$ m$/$s$.$ She observes an interface between two types of tissue. If the specific acoustic impedances of the tissues are z$=1\mathrm{.}42$ Pa s$/$m and z$=1\mathrm{.}68$ Pa s$/$m$,$ what percentage of the incident ultrasound intensity is reflected by the interface?