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Passing A Very Small Bridge

**What is the question?**
**F**or march of soldiers to pass any point, there must be **first** man crossing that point first and then later the** last** man should cross the same point.
Now the dog runs from the first to last man in **1** minute, then runs back to first man in 2 minutes and again back to last man in **1** minutes. That means the dog must be at the **same point** where he started journey from first man. In other words, the dog has run for **2** **minutes** in** both** the directions. Hence, he must be at the **same point** after** 4 minutes.**
Interestingly, the dog was at **first man** when started & after **4 minutes** ended at the **last man **of the march. Since dog is at the same point after **4 minutes,** the entire march of soldiers is crossed that point in 4 minutes as first and last man have crossed that point.
The same when applied to a very small bridge which can be assumed as a point, we can say that it requires **4 minutes** for soldiers to cross it.
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