To conclude our workshop, we return to the core flaw of the Chain Substitution Method. If a junior analyst reverses the sequence of calculation, the final results change! Why? Because of the 'Joint Effect' or 'Residual'—an overlapping area where both volume and price grew simultaneously. In this final challenge, students must calculate the exact monetary value of this mathematical overlap to understand why the Logarithmic method was invented to fix it. 

Roadmap to Solution:

Step 1: Look at the base data: Budget Q₀ = 1000, C₀ = $500. Actual Q₁ = 1200, C₁ = $550.

Step 2: Identify the absolute change in Volume (ΔQ). Subtract the budgeted quantity from the actual quantity (Q₁ - Q₀).

Step 3: Identify the absolute change in Price (ΔC). Subtract the budgeted price from the actual price (C₁ - C₀).

Step 4 (The Residual): Multiply these two absolute differences together (ΔQ × ΔC). This represents the "corner square" where both factors grew together.