The question
A train leaves City A heading east at 60 mph. Thirty minutes later another train leaves City B heading toward City A at 80 mph. Without knowing the distance, can you determine where they meet? Describe the reasoning.
Brain teasers
Logical Relay
Missing numbers do not stop you from mapping the meeting point.
Hints
Think in ratios: once the second train leaves, the two trains are closing the remaining gap together.
Answer
Yes. By the time the second train departs, the first train has already covered 30 miles. The remaining gap is closed at 140 mph (60 + 80). The share of that closing distance covered by each train is proportional to its speed, so the meeting point lies 4/7 of the remaining gap from City B and 3/7 from the first train. Even without the exact distance, the ratio of how far each train travels after the second leaves is fixed.