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Question

A boy goes from his house to school by bus at a speed of 20 km/h and returns back through the same route at a speed of 30 km/h. The average speed of his journey is:

A
25 km/h
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B
40 km/h
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C
20 km/h
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D
24 km/h
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Solution

The correct option is D 24 km/h
Let the total distance from the house to the school be x km. The speed of the bus to and from the school is 20 km/h and 30 km/h, respectively. We can calculate the time taken for going to the school and coming back, as follows:

Time taken for going =x/20 h
Time taken for coming back
=x/30h

Total time taken for the entire journey =(x/20+x/30)h
Total distance for the entire journey =x+x=2x
Formula for average speed = Total distance travelled / Total time taken

Therefore,average speed =(2x)/(x/20+x/30)
2x/(5x/60)=24km/h

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