A train covers a distance of 12 km in 10 minutes. If it takes 6 seconds to pass a telegraph post, what is the length (in metres) of the train?

Introduction / Context:
This question makes you calculate the length of a train using its speed over a known distance and the time taken to cross a fixed object, a telegraph post. It is a direct application of the relationship between speed, distance, and time that frequently appears in aptitude tests on trains.

Given Data / Assumptions:

• The train covers 12 km in 10 minutes.
• Time taken to pass a telegraph post = 6 seconds.
• Crossing a post means the distance travelled equals the length of the train.
• The train moves at constant speed in a straight line.

Concept / Approach:
From the 12 km in 10 minutes information, we can compute the speed in metres per second. After that, crossing the telegraph post is a simple distance = speed * time calculation where distance equals the train length. We convert units properly to keep everything consistent and then find the required length in metres.

Step-by-Step Solution:
Step 1: Convert 12 km to metres: 12 km = 12000 m. Step 2: Convert 10 minutes to seconds: 10 minutes = 600 seconds. Step 3: Speed of train in m/s = distance / time = 12000 / 600 = 20 m/s. Step 4: When crossing a telegraph post, distance travelled during the crossing time is equal to length of train. Step 5: Time to cross the post = 6 seconds. Step 6: Train length = speed * time = 20 * 6 = 120 m.

Verification / Alternative check:
We can recheck the logic quickly. A train that travels 12 km in 10 minutes indeed moves at 72 km/h, which converts to 72 * 5 / 18 = 20 m/s. Using 20 m/s for 6 seconds gives 120 m, which is a reasonable length for a train. The calculations match at each step, confirming the result.

Why Other Options Are Wrong:

• 90 m and 100 m: These are too short for a train moving 20 m/s over 6 seconds, and give times or distances that do not match the given data.
• 140 m: This implies the train would need 7 seconds at 20 m/s, not 6 seconds, to pass the telegraph post.

Common Pitfalls:
Errors usually arise from improper unit conversions, such as forgetting to convert minutes to seconds or kilometres to metres. Some learners also mistakenly treat the 12 km as the train length rather than a distance used only for speed calculation. Keeping units consistent and remembering that a fixed post involves only the entire train length is crucial.

Final Answer:
The length of the train is 120 m.