Question

What is the average speed of a car which travelled from a to b with onward speed 60km/hr and returned 90km/hr of the whole journey?

Answer 1

Answer:

Average speed = 72km/hr

Explanation:

Refer attached photograph.

Alternatively, you can use the following formula when the distance covered in both halves is same:

$avg \: speed = \frac{2 \times v1 \times v2}{v1 + v2}$

Answer 2

Answer:

Let the distance between a and b be ‘S’.

Therefore, total distance travelled = S+S=2S

From a to b  Speed = distance/time

Therefore, 60 = S/T1, Therefore T1 = S/60

From b to a  Speed = distance/ time

Therefore, 90 = S/T2, Therefore T2 = S/90

Average speed = Total distance covered/Total                Time Taken => 2S/(T1+ T2)=         = 2S/(S/60+S/90)                 = S/(3S/180+2S/180)               = (2Sx180)/3S+2S                  = (2SX180)/5S                               = 2x36                                             = 72

Therefore, Average Speed = 72km/hr

Explanation: