Question

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

Answer 1

Let x=total time

      y = total distance

Then:            (use T=D/R)

Time for first 1/3 of distance = ( (y/3)/ 10 )

Time for second 1/3 of distance = ( (y/3)/20 )

Time for third 1/3 of distance = ( (y/3)/60 )

Time for entire trip = x = ( (y/3)/10 ) + ( (y/3)/20 ) + ( (y/3)/60 )

                           180x = 6y +3y + y

                            180x = 10y

                               x = y/18

 

Rate for entire trip = D/T = y/x

                 =  y / (y/18)

                 =  y * (18/y)

                 = 18 km/hr


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