Two trains start at the same time from stations A and B, which are $600$ km apart, moving towards each other at $60$ km/hr and $90$ km/hr respectively. After how long will they meet?

**Principle.** When two objects move towards each other, their *closing* speed is the sum of their individual speeds. **Apply.** Closing speed $= 60 + 90 = 150$ km/hr. Time to close the $600$ km gap: $\dfrac{600}{150} = 4$ hours. **Why option D (6 h) is tempting.** $600 / 100$ (some 'average' of the two speeds) gives $6$ — but for closing gap, you add the speeds, not average them.