Question

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

Answer 1

Let the total distance travelled be x.
Let the total time taken be T.

Average speed = Total distance/Total time = x/T

First half,

v = 10 km/h
s = 1/3 x
t= x/3/10 = x/30

Second half,

v = 20 km/h
s = 1/3 x
t= x/3/20 = x/60

Third half,

v = 60 km/h
s = 1/3 x
t = x/3/60 = x/180

Now,

Time for entire journey, T = x/30 + x/60 + x/180

Or T = 6x + 3x + x/180

Or T= 10x/180

Or T = x/18

Now,
Average speed of the entire journey, Vav = Total distance/Total time

= x/x/18

= 18 km/h

Answer 2

Answer:

18 km/hr

Explanation :

Let the total distance travelled be x km Then total time taken = 10x/3+20x/3+60x/3=30x+60x+180x

=1806x+3x+x=18010x=18x hrs Then average speed = =TotaltimetakenTotaldistancetravelled

=x/18x km/hr = 18 km/hr