If a recipe uses sugar and flour in the ratio 2:5, how much flour for 6 cups of sugar?

This is a proportion problem. The ratio of sugar to flour stays fixed at 2:5 no matter how much you make. First see what you multiplied the sugar by: the ratio says 2 cups of sugar, but you have 6 cups, and 6 ÷ 2 = 3, so you scaled up by 3. To keep the ratio the same, multiply the flour part by the same number: 5 × 3 = 15. So you need 15 cups of flour. You can also set it up as 2/5 = 6/flour and cross-multiply: 2 × flour = 5 × 6 = 30, so flour = 30 ÷ 2 = 15. Both methods give 15 cups, which confirms the answer.