DEMONSTRATION 4 — Teaching Resource: Solids Balance in Blending AI-assisted self-initiated educational sample. Hypothetical inputs; not a client project or a record of measured production. Learning outcome: calculate the mass fraction of solids after blending, using conservation rather than an unweighted average. Problem: mix 10 kg of stream A at 20% solids with 30 kg of stream B at 40% solids. Assume no mass loss, reaction or evaporation. Worked solution: total mass = 10 + 30 = 40 kg. Solids from A = 10 × 0.20 = 2 kg. Solids from B = 30 × 0.40 = 12 kg. Total solids = 14 kg. Final mass fraction = 14/40 = 0.35, or 35% solids by mass. Board sequence: draw two input arrows and one output; label mass and composition; write total and solids balances; substitute values; check units. The answer lies between 20% and 40% and closer to 40% because B contributes more mass. Common error: (20% + 40%)/2 = 30% ignores the unequal masses. Practice: 15 kg at 12% plus 5 kg at 28%. Answer: (1.8 + 1.4)/20 = 16% solids. Assessment: students must show both balances and explain why the simple average fails. This sample illustrates instructional structure and quantitative checking.