Horse gallops 1 mile in 2 min and 35 sec

We are to find speed in km/h

Let us first convert 2 min and 35 sec in hours

2 min and 35 sec = 2 min + 35/60 min = 2 min + 7/12 min = 31/12 min = 31/(12x 60) hours

Now the horse covers in 2 min 35 sec distance = 1 mile

i.e., the horse covers in 31/(12x 60) hours distance = 1 mile

Or the horse covers in 1 hours distance = 1 x [ (12x60)/31] mile

But 1 mile = 1.61 km

Thus the horse gallops [ 12x60/31 ] 1.61 km in 1 hour

That is the speed in km / h