Beyond Fractions

The same cross-multiplication solves any proportion: if a/b = c/d, then ad = bc. That one rule powers far more than fraction problems.

• Currency: convert prices between two exchange rates.
• Map scales: turn a distance on paper into real kilometres.
• Medicine: when a nurse calculates the millilitres of a drug to draw from a patient's weight, this is the math doing the work.

Learn the butterfly once and you have quietly learned a tool that runs through everyday calculation.