Author note — to cover

Cover mixture density by totalling mass and totalling volume separately then dividing, never averaging the densities, including finding an unknown component's mass or volume from a stated mixture density. Alloy and liquid contexts, ratios of components included. Parts-based mixing without density is Ratio's mixing-and-concentrations. Both tiers, though exam demand is crossover-level. (R11)

• Fluency — direct questions on the density of a mixture, limited to this lesson's stated scope (4 questions).
• Application — use the same skill in unfamiliar or contextual questions without introducing a later method (4 questions).
• Check — questions built around the exclusions and likely misconceptions named in the lesson brief (4 questions).