Question

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 30 km h−1. What is the average speed for Abdul’s trip?

Answer 1

HEY!!

Average spped while he was going = 20km/h

Average speed while he was returning = 30km/h

Total average speed = u + v / 2

20+30/2

50/2 =25km/h

Answer 2

Given :

Method I:  

Let the distance traveled by Abdul from home to school = s km
time taken to reach the school=t1 sec
For Return journey , Abdul cover distance =s km
time=t2 sec
∴ Average speed for forward journey[home - school ] = Total distance/ Total time.
20 km/h =s/t1
∴t1=s/20 h ------eq(1)
 Average speed for backward journey[school -home] = Total distance/ Total time.
30 km/h =s/t2
t2=s/30h -----eq(2)
Average distance for entire journey = Total distance/ Total time
                                                         =(s+s)/[s/20 +s/30]
                                                         =2s/s[1/20+1/30]
                                                          =2x20x30/50
                                                           =24 km/hr
Shortcut method : If equal distance are covered with speeds v1 and v2 then average distance=2v1v2/v1+v2
                           = 2x20x30/50
                            =24km/hr

∴ The average speed for Abdul's trip is 24km/h