Question

A motorcyclist drives from place a to b with a uniform speed of 30km/hr and return backs with a speed of 20km/hr.find its average speed.

Answer 1

let the distance from A to B is d.
time for going from A to B
$\frac{d}{30} \: hr$
time for returning from B to A
$\frac{d}{20} \: hr$
total time
$\frac{d}{30} + \frac{d}{20} = \frac{5d}{60} = \frac{d}{12}$
average speed is
$\frac{total \: dstance \: covered}{total \: time \: taken} = \frac{d + d}{ \frac{d}{12} } = \frac{2d}{ \frac{d}{12} } = 24 \frac{km}{hr}$