Question

A body cover 20 kilometre with a speed of 40 km/h
and next 20 km with a speed of 30 km per hour calculate average speed

Answer 1

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 

Answer 2

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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.

HOPE SO IT WILL HELP......