Two horses start trotting towards each other, one from A to B and another from B to A. They cross each other after one hour and the first horse reaches B, 5/6 hour before the second horse reaches A. If the distance between A and B is 50 km. what is the speed of the slower horse?

If the speed of the faster horse be  $fs$ and that of slower horse be  $ss$ then

  $fs+ss=\frac{50}{1}=50$ 

and        $\frac{50}{s}-\frac{50}{f}=\frac{5}{6}$

Now, you can go through options.

The speed of slower  horse is 20km/h

Since,   20+30=50

and  $\frac{50}{20}-\frac{50}{30}=\frac{5}{6}$