for part a lets look at it this way.
the class has 16 girls and 12 boys 28 total people.
so the pobability of picking a boy the first week is
Now we have reduced the sample space( there is one less boy)
16 girls 11 boys 27 total students
Now we want to pick a girl so
Since we want both events to happen we multiply them together and get
Think about part b and post what you come up with