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19. A car travels the first one-thirddistance at a speed of 10 km/h,the next one-third distance at20 km/h and the last one-thirddistance at 60 km/h. Then, theaverage speed of the car is(a) 30 km/h(c) 18 km/h(b) 24 km/h(d) of these

Answer»

Let the total distance be 3x km. Now, average speed is given by

average speed = (total distance travelled) / (total time taken)

So, let times taken for the first, second and third halves of the journeys be t1, t2, and t3 respectively. Then,

t1 = x/10

t2 = x/20

t3 = x/60

So, average speed = x / (t1 + t2 + t3)

= x / (x/10 + x/20 + x/60)

= 3/ ((6 + 3 + 1)/60)

= (3×60)/10

= 18 km/hr

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