解题步骤

v\_{\text{avg}} = \frac{\text{distance traveled}}{\text{time elapsed}}

v\_{\text{avg}} = \frac{\Delta x}{\Delta t}

v\_{\text{avg}} = \frac{x\_{\text{final}} - x\_{\text{initial}}}{t\_{\text{final}} - t\_{\text{initial}}}

v\_{\text{avg}} = \frac{80-30}{3-2}

v\_{\text{avg}} = 50 \text{mph}