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, where $m_2 > m_1$. 2. **Understanding fixed and variable costs:** Total costs consist of fixed costs $FC$ (constant regardless of quantity) and variable costs which depend on quantity produced. Variable costs per unit are denoted as $VC_U$. 3. **Expressing total costs:** $$ TC_1 = FC + VC_U \times m_1 $$ $$ TC_2 = FC + VC_U \times m_2 $$ 4. **Subtracting the two equations to eliminate fixed costs:** $$ TC_2 - TC_1 = (FC + VC_U m_2) - (FC + VC_U m_1) = VC_U (m_2 - m_1) $$ 5. **Solving for variable costs per unit $VC_U$:** $$ VC_U = \frac{TC_2 - TC_1}{m_2 - m_1} $$ 6. **Conclusion:** The correct formula for variable costs per unit is $$ VC_U = \frac{TC_2 - TC_1}{m_2 - m_1} $$ This matches the third formula option given.