Proportional Reasoning

A recipe feeds four and uses 2 cups of flour. Tonight you're cooking for ten. How much flour? You could guess, or you could feel the ratio: 10 is two-and-a-half times 4, so you need two-and-a-half times the flour. One multiplication, no measuring cups piled up. That instinct — "scale both quantities by the same factor" — is proportional reasoning, and it runs recipes, maps, currency, and dosing.