In college, science and math courses assume facility with fractions--that is, with ratios and proportions.  During the past few decades, college science teachers have become aware that many of their students cannot use ratios, cannot convert between different units, do not understand simple proportions, such as Ohm's Law or Newton's Second Law of Motion, F = ma.

However, research done in the past few decades has shown that most students can usefully understand those mathematical abstractions.  The route to understanding lies in approaches to teaching and learning that have been rather rare until recently: "The process of science cannot be learned by reading, listening, memorizing, or problem-solving.   Effective learning [useful understanding] requires active mental engagement." (From note to students in "Physics by Inquiry," a product of the Physics Education Group, PEG, at the University of Washington.)

Active mental engagement means solving puzzles, resolving contradictions, "figuring things out for oneself."  So, here's a puzzle:

Many health brochures on obesity give a "formula" for calculating BMI.  You divide your weight in pounds by your height in inches and then divide by your height again.  The number you get is further multiplied by a number they give you. Caluclate that  number.