Given
A horse race 10 entries
One person owns 5 of these horses
Procedure
There are
N=10C5 ways of randomly choosing 5 horses out of 10.
where
N=10!/(5!5!)=252
[tex]\begin{gathered} N=\frac{10!}{5!5!} \\ N=252 \end{gathered}[/tex]We assume that
1. order does not count,
2. winning probabilities are equal among horses,
3. results are completely random.
Then the owner has one chance in 252 that his 5 horses are placed 1st to 5th.
1/252 = 0.0039