I have an exam tomorrow and I cant understand this kind of problems where they ask for the probability of a group of people within a group. For...

I have an exam tomorrow and I cant understand this kind of problems where they ask for the probability of a group of people within a group. For example:

Can someone help me understand this question:

5 students have been randomly chosen from a class of 20 students. Find the probability that:

a) At least 1 of them was born on Tuesday

b) At least 2 were born on the same weekday