The five numbers are exactly the same, and they are all randomly selected from 25 numbers.
There is only one possibility to pick out these five numbers at the same time, so C25 takes 5, that is.
The denominator is 25*24*23*22*2 1 and the numerator is 1*2*3*4*5 to get 53 130.
So the probability of winning the first prize is 1/53 130.