Test analysis: According to the significance analysis, the third group accounts for 40%, while the third group has 20 people, so the total number of samples is =

The conclusion is that the students whose test scores are rated as "B" are

(3) The probability that the two selected students happen to be a boy and a girl is

Comments: This question belongs to the basic examination of probability formula, which requires candidates to skillfully use basic knowledge to solve problems.