What is the probability that a policeman will catch all of the speeders in the pack of 10 cars if he

Here, the total number of possible events, n(S) = 10 because there are altogether 10 cars.
And the number of favorable events, n(F) = 3 because there are 3 speeding cars.

Now the probability that a policeman will catch all of the speeders, P(F) = n(F) / n(S)
= 3/10
Hope it will be helpful for you if it's correct. Anyway the method is same.