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The speed of a car is 90 km/h in the first hour and 60 km/h in the second hour. What is the average speed of the car for the given two hours?
72 km/h
75 km/h
80 km/h
85 km/h
75 km/h
Since the time interval for both speeds is equal (1 hour), simply calculate the arithmetic mean: (90+60)/2=150/2=75 km/h.
Speed in the first hour = 90 km/h, Speed in the second hour = 60 km/h.
Average Speed=Total TimeTotal Distance
Since the time interval for both speeds is equal (1 hour), simply calculate the arithmetic mean: (90+60)/2=150/2=75 km/h.
Many students incorrectly apply the harmonic mean formula 2v1v2/(v1+v2) when the time intervals are unequal, or mistakenly use the geometric mean instead of the simple average when time is constant.
Calculate distance for each interval
Since Distance=Speed×Time, for the first hour: d1=90×1=90 km. For the second hour: d2=60×1=60 km.
d1=90 km,d2=60 km
Calculate total values
Total distance is d1+d2=90+60=150 km. Total time is 1+1=2 hours.
Total Distance=150 km,Total Time=2 hours
Calculate average speed
Apply the average speed formula: Average Speed=150/2=75 km/h.
Vavg=2150=75 km/h
B is correct because the total distance covered in 2 hours is 150 km, resulting in an average speed of 75 km/h.
This concept of average speed is frequently tested in 'Trains and Boats' problems where you must distinguish between constant time intervals versus constant distance intervals.
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