A body covers 20km with a speed of 40km/h and next 20 km with a speed of 30km/h .Calclate average speed
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A body cover 20 kilometre with a speed of 40 km/h
time taken, t₁ = 20/40 = 1/2 h
It covered next 20 km with a speed of 30 km/h
time taken, t₂ = 20/30 = 2/3 h
total time taken = t₁ + t₂ = 1/2 + 2/3 = (3+4)/6 = 7/6 h
total distance traveled = 20+20 = 40km
average speed = total distance / total time
⇒ average speed = 40/(7/6) = 40×6/7
⇒ average speed = 240/7 km/h = 34.28 km/h
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Another shortcut:
You see that it travels 20km each time with different speed.
When distance travelled each time is same and it travels at speed x and y, average speed can be calculated as:
average speed = 2xy/(x+y)
using this, average speed = 2×30×40/(30+40) = 34.28 km/h
time taken, t₁ = 20/40 = 1/2 h
It covered next 20 km with a speed of 30 km/h
time taken, t₂ = 20/30 = 2/3 h
total time taken = t₁ + t₂ = 1/2 + 2/3 = (3+4)/6 = 7/6 h
total distance traveled = 20+20 = 40km
average speed = total distance / total time
⇒ average speed = 40/(7/6) = 40×6/7
⇒ average speed = 240/7 km/h = 34.28 km/h
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Another shortcut:
You see that it travels 20km each time with different speed.
When distance travelled each time is same and it travels at speed x and y, average speed can be calculated as:
average speed = 2xy/(x+y)
using this, average speed = 2×30×40/(30+40) = 34.28 km/h
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