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A train travels at 100 km/h for 1 hour and at 80 km/h for 2 hours. What is the average speed?
86.67 km/h
90 km/h
85 km/h
88 km/h
86.67 km/h
Calculate total distance by multiplying each speed with its respective time and divide by the sum of times.
Speed 1 = 100 km/h for Time 1 = 1 hour, Speed 2 = 80 km/h for Time 2 = 2 hours
Average Speed=Total TimeTotal Distance
Calculate total distance by multiplying each speed with its respective time and divide by the sum of times.
Students often wrongly calculate the average speed as the simple arithmetic mean of the speeds: (100+80)/2=90 km/h.
Calculate Total Distance
Distance is the product of speed and time. For the first part, distance is 100×1=100 km. For the second part, distance is 80×2=160 km. Total distance = 100+160=260 km.
Total Distance=(100×1)+(80×2)=260 km
Calculate Total Time
Add the time taken for both parts of the journey. Total time = 1 hour+2 hours=3 hours.
Total Time=1+2=3 hours
Compute Average Speed
Substitute total distance and total time into the average speed formula: 260/3≈86.67 km/h.
Average Speed=3260=86.67 km/h
A is correct because the average speed calculated from total distance divided by total time is 86.67 km/h.
In physics and quantitative aptitude, average speed is always defined as total distance divided by total time, never as the average of speeds unless time intervals are equal.
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