Question

A man while driving to school comPuts the average speed for his triped to be 20km/hr.On his return to trip his average speed is 30km/hr.So what is his average speed for his trip.

Answer 1

$\huge\bold{\gray{\sf{Answer:}}}$

$\bold{Explanation:}$

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.