Question

Abdul ,while driving to school, computes the average speedfor his trip to be 20km/h . On his return trip along the same route, there is less traffic and the average speed is 30km/h . What is the average speed for Abdul 's trip?

Answer 1

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