There are 25 horses and 5 lanes. You have no idea about which horse is better than other.
Find in minimum possible races, the first three fastest running horses.
We will have 5 races with all 25 horses
Let the results be
Work through a process of elimination:
Where u1 faster than u2 , u2 faster than u3 etc and
We need to consider only the following set of horses
We race u1,v1,x1,y1,z1
We get u1 as the fastest horse
We can ignore y1,y2,y3,z1,z2 and z3 automatically since those can not be in the top 3.
Now we left with
Race u2,u3,v1,v2 and x1 (x2,x3 is ignored since v1,x1 are faster than both, so obvious choices are u2,u3,v1,v2 and x1)
The first and second will be second and third of the whole set
So we need minimum of 7 races to find the 3 fastest horses.