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Bunuel
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How do you arrive at this step:

"The length that should be traveled by train is 1+0.125=1.125 kilometers;"

I understand the steps before and after that, just not this part - thanks!

0.125km ---------------------------------------------- 1km
------|------------------------------------------------|

In red is a train and in blue a tunnel. The whole train to run through the tunnel, it should travel the length of a train plus the length of the tunnel.
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I have edited the question and the solution by adding more details to enhance its clarity. I hope it is now easier to understand.
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I think this is wrong the total length must be 1km + 0.125*2 (because the trains last end must leave the tunnel)
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Official Solution:

A train consists of 6 carriages, each 20 meters in length and with a 1-meter gap between adjacent carriages. If the train travels at a constant speed of 60 km/h, how long does it take for the entire train to pass through a 1-kilometer-long tunnel?

A. \(1\frac{1}{8}\) minutes
B. \(1\frac{1}{2}\) minutes
C. \(1\frac{3}{4}\) minutes
D. 2 minutes
E. \(2\frac{1}{4}\) minutes


The total length of the train is \(6 * 20\) meters for the carriages, plus five 1-meter gaps, resulting in a length of \(6 * 20 + 5 = 125\) meters;

The distance the train needs to travel is the sum of the tunnel length and the train's length, which is \(1 + 0.125 = 1.125\) kilometers;

To calculate the time required, we use the formula: Time = Distance ÷ Speed, which gives us \(\frac{1.125}{60}\) hours, or by converting to minutes, we get \(\frac{1.125}{60} * 60 = 1.125 = 1\frac{1}{8}\) minutes.


Answer: A

I think this is wrong the total length must be 1km + 0.125*2 (because the trains last end must leave the tunnel)

You're wrong because the front of the train must pass the entire length of the tunnel, plus the length of the train itself, for the whole train to clear the tunnel. The distance the train needs to travel is the sum of the tunnel's length (1 kilometer) and the train's length (125 meters). You don't need to multiply the train's length by 2. The solution is correct: the total distance is 1.125 kilometers.
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I like the solution - it’s helpful.
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