Question

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

Answer 1

Answer:

Explanation:

Given :

Method Is :  

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.

Hope it helps you.