12th grade physics organic explainer video target for NEET 2026 in indian medical entrance exam---**Question Stem:**
Q23. A boy walks to his school at a distance of 6 km with constant speed of 2.5 km/h and walks back with a constant speed of 4 km/h. His average speed for round trip expressed in km/h, is
**Options:**
A. 24 / 13
B. 40 / 13
C. 3
D. 1 / 2
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Let's solve this average speed problem step by step. A boy walks to school which is 6 kilometers away at a constant speed of 2.5 kilometers per hour. Then he walks back home at a different speed of 4 kilometers per hour. We need to find his average speed for the entire round trip.
Now let's calculate the time taken for each part of the journey. For the trip to school, we use the formula time equals distance divided by speed. So time to school equals 6 kilometers divided by 2.5 kilometers per hour, which equals 12 over 5 hours or 2.4 hours. For the return trip, time equals 6 kilometers divided by 4 kilometers per hour, which equals 3 over 2 hours or 1.5 hours.
Now let's calculate the total distance and total time for the round trip. The total distance is simply 6 kilometers plus 6 kilometers, which equals 12 kilometers. For the total time, we need to add the two time intervals: 12 over 5 hours plus 3 over 2 hours. To add these fractions, we need a common denominator of 10. So we get 24 over 10 plus 15 over 10, which equals 39 over 10 hours or 3.9 hours.
Now we can calculate the average speed using the formula: average speed equals total distance divided by total time. Substituting our values, we get 12 kilometers divided by 39 over 10 hours. When dividing by a fraction, we multiply by its reciprocal, so this becomes 12 times 10 over 39, which equals 120 over 39. We can simplify this fraction by dividing both numerator and denominator by 3, giving us 40 over 13 kilometers per hour. This matches option B in the multiple choice answers.
The correct answer is option B: 40 over 13 kilometers per hour. This is approximately 3.08 kilometers per hour. Remember the key concept: average speed is not the same as the average of the speeds. A common mistake would be to simply add 2.5 and 4, then divide by 2, giving 3.25 kilometers per hour, which is incorrect. For problems involving round trips with different speeds, always use the formula: average speed equals total distance divided by total time. This ensures you get the correct answer for NEET physics problems.