Question

71. Kiran covers a certain distance at 80 km/h and
returns back to the same point at 20 km/h.
Then the average speed during the whole
journey be
(A) 28 km/h
(B) 32 km/h
(C) 35 km/h
(D) 30 km/h​

Answer 1

Answer:-

Let the distance travelled by Kiran be d km.

Distance travelled in 1st interval

= Distance travelled in 2nd interval = d km.

Given:

Speed of kiran for 1st interval = 80 km/h

We know,

Time = distance/speed

Time taken to travel in 1st interval = (d/80) hrs.

Similarly,

Time taken to travel in 2nd interval = (d/20) hrs

We know that,

Average speed = Total Distance travelled/total Time taken.

• Total distance travelled = d + d = 2d km.

• Total Time taken = (d/80) + (d/20) = (20d + 80d) / (1600)

→ Total Time taken = 100d/1600 = (d/16) hrs

Hence,

Average speed of Kiran = 2d/ (d/16)

→ Average speed of Kiran = 2d * (16/d)

→ Average speed of Kiran = 32 km/h

Therefore, the average speed of Kiran is 32 km/hr (Option - B).

Answer 2

To Find :

• we have to find the average speed of Kiran during Whole journey.

Solution :

Kiran covers a certain distance with the speed of 80km/h

And he returns back with the speed of 20km/h

As we know that,

$\boxed{\green{\bf{Speed = \frac{Distance}{Time}}}}$

And ,

• Time = Distance/speed

Time taken to cover a Certain distance with the speed of 80km/h = x/80

Time taken to cover a Certain distance with the speed of 20km/h = x/20

$\blue{\bf{Average\: speed = \frac{Total\: distance}{Total\:time\:taken}}}$

Total time taken = x/80 + x/20

⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀ = 5x/80

⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀ = x/16

Total distance covered = x + x

⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀ = 2x

Average speed = 2x/(x/16)

⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀= 2x × 16/x

⠀⠀⠀⠀⠀⠀⠀⠀⠀ = 32

$\large\dag \large { \red{\underline{\bf{Hence }}}}$

$\purple{\textrm{Speed of Kiran is 32km/h}}$

• Option (B) is correct✓

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