Question

A car is moving on a straight road covers one third of the distance with a speed of 20 km/h and the rest with
a speed of 60 km/h. The average speed of the car is
(1) 40 km/h
(2) 50 km/h
(3) 36 km/h
(4) 55 km/h
​

Answer 1

36Km/hr
Explanation:

Let the distnace travelled by a car on staright path be x.
The one thrid distance is covered with speed of 20 km/h and the rest two- third distance with speed of 60 km/h
Thus,
time required for the one third of the journey is
fraction numerator begin display style bevelled x over 3 end style over denominator 20 end fraction equals x over 60
The time required for the rest part of the journey is
fraction numerator begin display style bevelled fraction numerator 2 x over denominator 3 end fraction end style over denominator 60 end fraction equals x over 90
Thus the total time taken to complete the journey is,
x over 60 plus x over 90 equals x over 36
Thus, the average speed is,

Average speed = Total distance travelled/ total time taken
A v e r a g e space s p e e d space equals space fraction numerator x over denominator begin display style bevelled x over 36 end style end fraction
T h u s space a v e r a g e space s p e e d space space equals 36 space k m divided by h space