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Train and tunnel

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A train of length $200$ m moves at constant speed and passes a signal pole in $10$ s. How many seconds does it need to cross a tunnel of length $300$ m, counting from the moment its front enters until its rear leaves?

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Hint

Passing the pole means covering exactly the train's own length.

Solution

The train passes the pole by covering its own length $200$ m in $10$ s, so its speed is $20$ m/s.
To clear the tunnel, the train must travel the tunnel length plus its own length: $200 + 300 = 500$ m.
The time is $500 \div 20 = 25$ s.
The answer is $25$ .

Answer: $25$

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