The average score of students in Class A is 85.
The total score is equal to 85× 7 = 595. So x=5.
The median score of students in Class B is 80+y=83, and Y = 3.
(2) The scores of seven students in Class A are 78, 79, 80, 85, 85, 92 and 96 respectively.
The average score of seven students in Class A is. x=85。
The variance of seven students' grades is17 (49+36+25+0+49+121) = 40.
(3) At least one student in Class A is event A,
The opposite event is that two students are randomly selected from students with 90 points or above, and there are no students in Class A;
From the root, stem and leaf map, A's score was higher than 90 twice and B's score was higher than 90 three times.
Two grades were randomly selected from the grades of Class A and Class B, with 5×4 cases, but 3×2 times were not in Class A..
Then p (a) =1-3× 25× 4 = 710.