Manish drive to his office at 20km/hr. After reaching office, he realize that today is a holiday of 'Labor Day'.He went back at average speed of 30km/hr. Discounting the time spent in the stoppage what was his average speed of his journey ?

24km/hr (not 25km/hr which might be guessed by many)

D : distance traveled

T1 : time of going to office

T2 : time of returning back

Y : Average Speed

D = 20 * T1

T1 = D/20

D = 30 * T2

T2 = D/30

2D = Y(T2 + T1)

2D = Y(D/20 + D/30)

2D = Y(3D/60 + 2D/60)

2D = Y(5D/60)

Y = 120D/5D

=> Y = 24