Question 3

Abdul, while driving to school, computes the average speed for his trip to be 20 km h^{-1}. On his return trip along the same route, there is less traffic and the average speed is 40 km h^{-1}. What is the average speed for Abdul’s trip?

Answer

**Case I: **While driving to school

Average speed of Abdul’s trip = 20 km/h

Total distance = *d*

Let total time taken = *t*1

Speed = Distance / time

20 = d / t_{1}

t_{1} = d / 20…(*i*)

**Case II:** While returning from school

Total distance = *d*

Speed = 30 km/h

Now,total time taken = t_{2}

Speed = distance / time

30 = d / t_{2}

t_{2} = d / 30 ..... (ii)

Total distance covered in the trip = *d* + *d* = 2*d*

Total time taken, *t* = Time taken to go to school + Time taken to return to school

= *t*_{1} + *t*_{2}

Total time taken = d / 20 + d / 30

= 3d + 2d / 60

= 5d / 60

= d / 12

From equations (i) and (ii),

Average Speed = 120/5

= 24 km/h

Hence, the average speed for Abdul’s trip is 24 km/h.

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