Christopher and his parents live $115$ miles apart. They drive toward each other and meet at a restaurant. Christopher drives $1.5$ hours; his parents drive $1$ hour. Christopher's speed is $10$ mph faster than his parents'. What is the **parents'** speed?

Let $r$ be the parents' speed. Then Christopher's speed is $r + 10$. Because they drive toward each other, the sum of their distances equals the gap between their homes: $1.5 (r + 10) + 1 \cdot r = 115$. Distribute: $1.5 r + 15 + r = 115 \Rightarrow 2.5 r = 100 \Rightarrow r = 40$. So the parents drive at $\boxed{40}$ mph, and Christopher at $50$ mph. Check: Christopher covers $50 \times 1.5 = 75$ miles; parents cover $40 \times 1 = 40$ miles. Total $= 115$ ✓. - Trap C ($50$) is Christopher's speed. - Trap A and D miss the equation. The key move is recognising that *toward each other → distances **add** to the gap*, vs. *same direction → distances **subtract** from each other*.