Proportions

A bake sale prices 2 cookies at $1. You want 6 cookies — how much do they cost? You don't need a calculator to feel that the answer is $3. But why? Because the price-per-cookie hasn't changed: 2 for $1 is the same deal as 6 for $3. You've set up two equal ratios — and that, with a name and a method, is one of the most useful tools in all of arithmetic.