1. **Stating the problem:** We have total costs $TC_1$ for producing $m_1$ units and total costs $TC_2$ for producing $m_2$ units, with $m_2 > m_1$. Total costs consist of fixed costs $FC$ and variable costs proportional to quantity produced. 2. **Formula and rules:** Total costs can be expressed as $$TC = FC + VC_U \times m$$ where $VC_U$ is the variable cost per unit. 3. Since fixed costs $FC$ do not depend on quantity, subtracting total costs at two production levels eliminates $FC$: $$TC_2 - TC_1 = (FC + VC_U \times m_2) - (FC + VC_U \times m_1) = VC_U (m_2 - m_1)$$ 4. Solving for $VC_U$: $$VC_U = \frac{TC_2 - TC_1}{m_2 - m_1}$$ 5. **Explanation:** This formula calculates the variable cost per unit by dividing the change in total costs by the change in quantity, effectively isolating the variable cost component. 6. **Checking other options:** - $\frac{TC_2 + TC_1}{m_2 + m_1}$ averages total costs and quantities, not isolating variable costs. - $\frac{m_2 + m_1}{TC_2 + TC_1}$ inverts costs and quantities, not meaningful here. - $\frac{TC_2 \cdot TC_1}{m_2 \cdot m_1}$ multiplies costs and quantities, unrelated to variable cost per unit. **Final answer:** $$VC_U = \frac{TC_2 - TC_1}{m_2 - m_1}$$