Ultrasound has a frequency above the upper human hearing limit. It is partially reflected at boundaries between different media, allowing hidden structures to be detected. For a pulse travelling to a boundary and back, distance = wave speed × return time ÷ 2; the measured time includes both journeys. Rearrange the relationship before substituting if a different quantity is required. A correct answer needs an appropriate unit as well as a numerical value. Explain the result. Check your reasoning.