Question

A train travels at a speed of 60 km/hr for 0.52 hr, at 30 km/h for the next 0.24 hr and then, at 70 km/h for the next 0.71 hr. What is the average speed of the train ?

CBSE Class IX Science SA 2 (3 Marks)

Answer 1

(i) In the first case, the train travels at a speed of 60 km/h for a time of 0.52 h.
Now,          Speed = Distance ÷ Time = 60 km/h
 Distance = 60 x 0.52 = 31.2 kin                          ....(1)
(ii) In the second case, the train travels at a speed of 30 km/h for a time of 0.24 h.
 Now,            Distance = Distance ÷ Time = 30 km/h
                    Distance = 30 x 0.24 = 7.2 km                            .....(2)
 (iii) In the third case, the train travels at a speed of 70 km/h for a time of 0.71 h
Now,                     Speed = Distance ÷ Time = 70 km/h
                          Distance = 70 x 0.71 = 49.7 km                  ......(3)     
From equation 1, 2 & 3, we get,
                 Total distance travelled = (31.2 + 7.2 + 49.7) km = 88.1 km
                   Total time taken = (0.52 + 0.24 + 0.71) h = 1.47 h
                 Average speed = Total distance travelled ÷ Total time taken
                                               = (88.1 ÷ 1.47)
Hence, average speed
                                               = 59.9 km/h

Answer 2

Answer:

59.9 km/h

Explanation:

in this problem, first of all, we have to calculate  the distances travelled by  train under 3 different conditions of speed and time

1)speed=distance/time

=60=distance/0.52

=60*0.52=distance

=31.2 km

in first case distance of train is 31.2 km               (1)

2)speed=distance/time

=30=distance/0.24

=30*0.24=distance

=7.2 km

in second case distance of train is 7.2 km            (2)

3)speed=distance/time

=70=distance/0.71

=70*0.71=distance

=49.7 km

in third  case distance of train is 49.7 km                (3)

from equations 1,2and 3

total distance=31.2+7.2+49.7=88.1 km

total time=0.52+0.24+0.71=1.47 hr

average speed=total distance/total time taken

=88.1/1.47

=59.9 km