Question 621048
Let the distance between A and B = 'D'
Time taken while traveling from A to B with a speed of 30kmph = D/30
Time taken while traveling from B to A with a speed of 60kmph = D/60
.
Average speed = (Total Distance Traveled)/(Total time taken) 
Average speed = {{{(D+D)/((D/30)+(D/60))}}}
Average speed = {{{(2D)/((2D+D)/60)}}}
Average speed = {{{(2D*60)/3D}}}
Average speed = 40kmph