First of all, I'm sorry if title was not clear, I thought about it for 5 minutes and I couldn't make it better. (English is not my first language)
Ok, now the task is the following. In one class, there are 20 girls and 12 boys. On a revision, the teacher choose with the same probability for each student, one by one, three students. What is the probability the chosen student to be: a) male, b)female if the teacher has chosen 2 girls firstly?