Question

A boy runs for 10 minutes at a uniform speed of 18 km per hour. At what speed should he runs for the next 20 minutes so that the average speed becomes 12 km per hour?

Answer 1

Given,

$v_1=18km/hr$

$t_1=10min=10/60=1/6hr$

$t_2=20min=20/60=1/3hr$

Distance coverd in first part of journey

$s_1=v_1\times t_1=18\times \frac{1}{6}=3km$

Average speed = $12km/hr$

Total time = $t=t_1+t_2$

$t=1/6+1/3=1/2hr$

Total distance = $s=v\times t=\frac{1}{2}\times 12=6km$

Distance coverd in second part of the journey

$s_2=s-s_1$

$s_2=6-3=3km$

Therefore, $v_2=\frac{distance}{time}=\frac{s_2}{t_2}=\frac{3}{\frac{1}{3}}=9km/hr$.