Problem

乙、計算、證明題：共 3 大題，每大題 12 分，共 36 分。須詳細寫出計算及證明過程，否則不予計分。

1. An airplane is flying on a flight path that will take it directly over a radar tracking station. The distance $s$ is decreasing at a rate of 640 kilometers per hour when $s=16$ km. What is the speed of the plane?