There are 25 horses and only five tracks in a race.
How do you find the second coming horse of all the 25
horses, provided there is no stop clock? (obviously, a
horse cannot participate more than once in a race).

Divide the set of 25 horses into 5 non-overlapping sets of 5
horses each. Have a race each for all the horses in each
set. This makes it a total of 5 races, one for each set.

Now, have a race for the winners of each of the previous 5
races. This makes it a total of 6 races.

Observe the position of each horse in the 6th race and
correspondingly number the sets. i.e. the set of the winner
of 6th race will be said to be set no. 1 while the set of
the loser of the 6th race will be said to be set no. 5.

Now, possible candidates for the first three positions
exclude the followings:
1. Any horse from set 4 or set 5.
2. Any horse except the winner from set 3,.
3. Any horse except the winner and the 2nd position from set 2.
4. Any horse except the winner, 2nd position and 3rd
position from set 1.

So now we have 6 candidates for top 3 positions. However, we
know that the winner of set 1 is the fastest horse in the
whole group of 25 sets.

So now we have 5 candidates for the second and third
position. What better way to find out who's who than to have
a race of these 5 horses. Race them and this will solve our
problem in just 7 races.

LAST TWO CAN BE REMOVED FROM EACH GROUPS.
WE HAVE LEFT WITH 9 HORSES.

6 RACE(F) A1->F1 B1->F5 C1->F2 D1->F3 E1->F4

SUPPOSE (CAN BE GENERIC,ANALYS) FROM RACE(F)

NOW F1(A1) IS FASTES HORSE AMONG 25.
ALL HORSES FROM GORUP B AND E CAN BE ELIMINATED(since E1
and B1 is at 4th and 5th position respectivly). AND ANALYS
A BIT, C3, D2 AND D3 ALSO CAN BE ELIMINATED (since in any
senario C3 will max come 4th, D2->4th and D3->5th). NOW
LEFT WITH 5 HORSES.

5X5 = 25...
make group of 5 and total 5 such groups ...
race each grup at a time... select winner from each group.
race these 5 winners ... and mark them as their positions

best case: the fastest horse and the 2nd coming horse are
in two different group. Then we can simply say the 2nd
coming horse in the winners' race is the answer.

worst case: the fastest horse and the 2nd coming horse are
in same group. So any from the four except the winner from
the fastest horse's group (say 1st group) may be the 2nd
coming horse.

so need another race.... 4 from 1st group (except winner)
and winner of 2nd group .... who wins is 2nd best.

I was asked this question in microsoft.
The correct answer to find out top 3 horses is 7.
Explaination of Desinerd i.e. Answer#1 seems the most
appropriate.

Run each horse in a race, always keeping the top two to
compete in the next race, until the last race in which the
top two are identified. So run 8 races instead of 7,
sometimes the simple solution is the best.

Obviously a horse can't run twice in a race. Sometimes when
something is too obvious it makes you think it's a trick
statement.

The first soln is correct, but I think its not understandable.
and @ Animesh Sonkar, your soln is correct until 6th race.
In the 7th race, u have eliminated the first rank, the fouth
nd the fifth. But u have raced only 4 horses.. that is whr u
missed. Correct Soln.:-

The fifth horse in the seventh race would be rank 2 horse of
the group which has 2nd rank in the fifth (all winners) race.
So, all the scenarios would be taken care of now.

Eg. after 5th race, let the positions be:

A1 B1 C1 D1 E1 (in order of rank)

now A1 is the fastest.--> eliminate it
D1 nd E1 can't be 2nd nd 3rd.(!!!)
Now we remain with B1 nd C1.

The other horses in the race would be A2 A3 and B2.

So, in every possible case, we can get the first three
positions confirmed.
* We don't need B3 because, B1 nd B2 are already faster than
it (evn after leaving A1), therefore, it can't be 3rd.

* We don't need horse C2 because, B1 nd C1 are already
faster than C2, therefore it is not the contender of top
three positions.

Obviously horses must be allowed to compete in more than one
race, and they are assumed not to tire as they run races, so
their performance is constant.

Round 1: 5 races of 5
Round 2: 5 winners of Round 1
-> winner is overall 1st place (6 races)
Round 3: 2nd and 3rd places from Round 2,
plus horses that came 2nd & 3rd behind Round 2 1st
placer in
Round 1
plus horse that came 2nd behind Round 2 2nd placer in
Round 1
-> winner is 2nd place overall
-> 2nd place is 3rd place overall

So you can find the winner in 6 races (trivial) and top
three in 7 races.

You cannot simply take the fastest horse from each group of
five. You have to look at the times of all the horses and
take the five fastest times from all 25 and then select the
top 5. Some would argue length and turf play in, but all
else equal, the fastest horse of one race could be slower
than the slowest of the other 4 races, so the winner of
each race is not a good answer.

Scientist decided to do a study on the population growth of
rabbits. Inside a controlled environment, 1000 rabbits were
placed.
Six months later, there were 1000Z rabbits. At the beginning
of the 3rd year, there were roughly 2828Z rabbits, which was
4 times what the scientists placed in there at the beginning
of the 1st year.
If Z is a positive variable, how many rabbits would be there
at the beginning of the 11th year?

A person wanted to withdraw X rupees and Y paise from the
bank. But cashier made a mistake and gave him Y rupees and
X paise. Neither the person nor the cashier noticed that.
After spending 20 paise, the person counts the money. And
to his surprise, he has double the amount he wanted to
withdraw. Find X and Y.

Three men - Sam, Cam and Laurie - are married to Carrie,
Billy and Tina, but not necessarily in the same order.
Sam's wife and Billy's Husband play Carrie and Tina's
husband at bridge. No wife partners her husband and Cam
does not play bridge.
Who is married to Cam?

There were two men standing on a street. The one says to the
other, "I have 3 daughters, the product of their ages is 36.
What is the age of the OLDEST daughter?"
The second guy says, "I need more information." So, the
first guy says, "The sum of their ages is equal to the
address of the house across the street."
The second guy looks at the address and says, "I still need
more information." So, the first guy says, "My oldest
daughter wears a red dress."

Two people enter a race in whick you run to a point and
back. Person A runs 20 mph to and from the point. Person B
runs to the point going 10 mph and 30 mph going back.
Who came in first?

Mrs. Watsherface had a garage sale. A custmer named Gina
bought an old lamp and a rug. She paid a total of $5.25 for
everything. The rug cost 25 cents more than the lamp.
How much did each cost?

Montu, Bantu, Chantu and Pintu have pets.
Montu says, "If Pintu and I each have a dog, then exactly
one of Bantu and Chantu has a dog."
Bantu says, "If Chantu and I each have a cat, then exactly
one of Montu and Pintu has a dog."
Chantu says, "If Montu and I each have a dog, then exactly
one of Bantu and Pintu has a cat."
Pintu says, "If Bantu and I each have a cat, then exactly
one of Bantu and I has a dog."
Only one of the four is telling the truth. Who is telling
the truth?

Everyday in his business a merchant had to weigh amounts
from 1 kg to 121 kgs, to the nearest kg. What are the
minimum number of different weights required and how heavy
should they be?