You have 25 horses and you have to pick 3 fastest horses.
1. There are only 5 tracks.
2. In each track only one horse can run. So you can conduct each race with 5 horses only.
3. you don't have timer or stop watch.
How many minimum number of races can you conduct to pick 3 fastest horses?

Answer is 7.
Step1: Devide horses into 5 groups and Conduct race for each group. arrange them based on their Position as shown in figure.

Step2: Take fastest horse from each group conduct a race among them and assume that positions are as shown in below figure.

Step3: Arrange the groups as shown in figure based on their position.

Now H01 horse is the fastest horse.
Group4 and Group5 are not eligible for top 3.
H12 and H13 are not eligible for top3 because they are slower than H11(which is in top3).
H08 is not eligible for top3. because it is slower than H07(which is top3).

Step4: Now H02,H03,H06,H07 and H11 are in top2 and top3 position.
Conduct race among those and pick top2 horses.

So top 3 horses are H01 and top2 horses picked from step4.