Question

Jai drives from city a to city b at an average speed of 90kmph. As soon as he reaches, he turns around and comes back to city a, but this time at an average speed of 30kmph. What is is average speed in kmph for the whole journey? Round off your answer to the nearest whole number.

Answer 1

average speed =1/(((1/v1)+(1/v2))/2)= 45kmph.

Answer 2

Given:

• 90 km/hr from a to b
• 30 km/hr from b to a

To find:

• Average speed for whole journey?

Calculation:

The average speed is defined as the ratio of total distance and total time.

• Now, the total distance will be d + d = 2d, when d is the distance from a to b.

$\therefore \: avg. \: v = \dfrac{distance}{time}$

$\implies \: avg. \: v = \dfrac{2d}{t}$

$\implies \: avg. \: v = \dfrac{2d}{ \dfrac{d}{90} + \dfrac{d}{30} }$

$\implies \: avg. \: v = \dfrac{2}{ \dfrac{1}{90} + \dfrac{1}{30} }$

$\implies \: avg. \: v = \dfrac{2}{ \dfrac{1 + 3}{90} }$

$\implies \: avg. \: v = \dfrac{2}{ \dfrac{4}{90} }$

$\implies \: avg. \: v = \dfrac{90}{ 2 }$

$\implies \: avg. \: v = 45 \: km/hr$

So, average velocity for whole journey is 45 km/hr.