A train runs at 50 km/h when moving (no stoppage) but averages 40 km/h including stops. On average, how many minutes per hour does the train stop?

Introduction / Context:
Overall average speed over a clock hour equals distance covered in that hour. If the running speed is higher, the train must have stopped for some fraction of that hour. Solve for the moving-time fraction, then convert the remainder to minutes.

Given Data / Assumptions:

• Running speed = 50 km/h.
• Overall speed = 40 km/h across 1 clock-hour.

Concept / Approach:
Let moving time per hour be t hours. Then distance in one clock-hour = 50t. Given overall average 40 km/h, 50t = 40 ⇒ t = 0.8 h.

Step-by-Step Solution:

Verification / Alternative check:
In 48 minutes at 50 km/h, distance is 40 km, matching the overall 40 km/h.

Why Other Options Are Wrong:
6, 10, 14, 18 minutes correspond to incorrect moving fractions.

Common Pitfalls:
Confusing arithmetic mean of speeds with time-weighted average; not solving for t first.

Final Answer:
12 min