I keep on having problems with the same kind of questions involving combinatorics. This is one such problem where I am stuck.
A bag contains 9 discs labelled 1 to 9. Andy chooses 4 discs at random, without replacement.
Find the probability that:
a) the 4 digits include at least 3 odd digits
b) the 4 digits add up to 28
I started by working out that there are 5 odd digits and 4 even digits, and that there are $^9P_4$ different numbers the 4 digits can form. But now I don't know where to go from here. Please also describe a way of going about these kinds of questions. Thanks!