Algorithms, Discrete Mathematics, Horses, Horse Races, Riddles.
first 25 races, lets assume these are the results. We can eliminate the 4th and 5th horses from each race. The reason we cannot yet eliminate the 2nd and 3rd ones yet is that any single race could produce the top 3 fastest horses of all 125. Lets pick the winners of these to form the next 5 races.