FIND THE AVERAGE

$average~speed~for~the~whole~trip=\dfrac{total~distance~traveled}{total~time~elapsed}$

We know that $t=\dfrac{d}{v}$ where $t$ = time, $d$ = distance and $v$ = speed. So

$average~speed~for~the~whole~trip=\dfrac{d+d}{t_{(A-B)}+t_{(B-A)}}$

$60=\dfrac{2d}{\dfrac{d}{50}+\dfrac{d}{v}}$

$60=\dfrac{2d}{\dfrac{vd+50d}{50v}}$

$60=\dfrac{2}{\dfrac{v+50}{50v}}$

$60=2\left(\dfrac{50v}{v+50}\right)$

$60=\dfrac{100v}{v+50}$

$60v+3000=100v$

$40v=3000$

$v=75~kph$

Let d be the distance between A and B. T1 = d / 50 : travel time from A to B. Let S be the speed from B to A. T2 = d/S : travel time from B to A. 60 = 2d/(T1 + T2) : average speed for the whole trip. 60 = 2d/(d/50 + d/S) : substitute T1 and T2. S = 75 km/hour : solve the above equation for S.

easy one ! studied this one in ninth class

Let, $x$ km/ hr be the average speed to travel from B to A.

Now, $60=\frac{2*50*x}{50+x}$

or, $5x=3x+150$

or, $x=\boxed{75}$

nice question