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Puzzle: Horsies

I feel in a puzzle-giving mood, so here's one that rcp will recognize this from her Oracle interview:

There are 25 horses, and you can race 5 of them at a time. Strangely, you have no stopwatch, but the horses always run exactly the same in every race. How many races does it take to figure out:
* the fastest horse?
* the top three fastest?

Comments (4)

NK:

1) 5 races of 5 horses each - Find the top three(I,II,III) from each group.
2) 6th Race - Pick Fastest (I) horse from each group and run a race among these 5 fastest horses.
- This should give out the fastest horse.
3) 7th race - From step 2, pick the 2nd ranking horse from the group which had the fastest horse and the remaining fastest horses (I) which took part in the race as part of step 2.
Run a race among these, which should give out 2nd fastest horse.
4) 8th race - Pick the 3rd horse from the group which won the 2nd fastest place and run the race among remaining horses in step 3. This step should give the 3rd fastest horse.


Feng:

Maybe only 7 races needed.
First 6 races same as above with A-E groups.
The 7th race take A2,A3,B1, B2,C1 and the first two are picked. No other horses in C,D,E are relevant because they can not be faster than A1,B1,C1.

prabhakar:

only 7 races needed

Dinesh Taparia:

Hi....
There r minimum of 6 races to find out the fastest horse. But I don't know how it can be.

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