Here’s the problem:
An airplane is flying at 36,000 feet directly above Lincoln, Nebraska. A little later the plane is flying at 28,000 feet directly above Des Moines, Iowa, which is 160 miles from Lincoln. Assuming a constant rate of descent, predict how far from Des Moines the airplane will be when it lands. 
Question: Which student demonstrates better understanding? Why?
 Problem courtesy of Phillips Exeter Academy. Hat Tip: Alison Blank