Question

A car travel 1/3rd distance on a straight road with a velocity of 10 km/hr,next 1/3rd with velocity 20km/hr and the last 1/3 rd with a velocity 60km/hr.what is the average velocity of the car in the whole journey

Answer 1

let total daistance =S
average velocity=total displacement/total time
time = s/v
calculate three different time periods 
you will get average velocity=S/your total calculated time

Answer 2

average velocity = total distance / total time taken
also
Time = distance / speed

average velocity =
$\frac{ \frac{1}{3} + \frac{1}{3} + \frac{1}{3} }{ \frac{ \frac{1}{3} }{10} + \frac{ \frac{1}{3} }{20} + \frac{ \frac{1}{3} }{60} } \\ \frac{1}{ \frac{1}{30} + \frac{1}{60} + \frac{1}{180} } \\ \frac{1}{ \frac{6 + 3 + 1}{180} } \\ \frac{1}{ \frac{10}{180} } \\ \frac{180}{10} \\ 18 \: \frac{km}{hr}$

Average velocity = 18 km/hr