Without any stoppage, a person travels a certain distance at an average speed of 42 km/h. Including regular stoppages, the same distance is covered at an average speed of 28 km/h. How many minutes per hour, on average, does the person stop?

Introduction / Context:
This question examines the relationship between running speed, effective average speed and stoppage time. It is a standard speed and time problem used to test understanding of average speed over a journey with interruptions.

Given Data / Assumptions:
- Running speed without stoppage = 42 km/h.
- Effective average speed including stoppages = 28 km/h.
- The distance travelled in both cases is the same.
- We are asked to find the stoppage time per hour of journey.

Concept / Approach:
In one hour of clock time, if the person never stops, he would travel 42 km. However, due to stoppages, in one hour of clock time he actually covers only 28 km. We can find how much time in each hour he is actually moving at 42 km/h, and the remaining time is stoppage time.

Step-by-Step Solution:
Step 1: Let in 1 hour of actual clock time, the person travels 28 km including stoppages.Step 2: While he is moving, his speed is 42 km/h. Let the moving time per hour be t hours.Step 3: Distance covered while moving in that hour is 42 * t kilometres.Step 4: This must equal the effective distance covered in one hour, which is 28 km. So 42 * t = 28.Step 5: Solve for t: t = 28 / 42 = 2 / 3 hour.Step 6: Total clock time per hour is 1 hour, so stoppage time per hour = 1 - 2 / 3 = 1 / 3 hour.Step 7: Convert 1 / 3 hour to minutes: (1 / 3) * 60 = 20 minutes.

Verification / Alternative check:
If he moves 40 minutes out of every 60 minutes at 42 km/h, the distance covered is 42 * (40 / 60) = 42 * (2 / 3) = 28 km. This matches the given effective average speed of 28 km/h, confirming that stopping for 20 minutes each hour is correct.

Why Other Options Are Wrong:
- 15 minutes and 18 minutes yield different effective average speeds when you compute 42 multiplied by the corresponding fraction of the hour, not 28 km/h.
- 22 minutes likewise does not produce the exact ratio needed to reduce 42 km/h down to 28 km/h.

Common Pitfalls:
- Taking the ratio 28 : 42 and interpreting it incorrectly as time ratio instead of using the concept of distance = speed * time.
- Confusing total journey time with stoppage time per hour.
- Forgetting to convert the final time from hours into minutes.

Final Answer:
The person stops for 20 minutes per hour on average.