Analysis of test questions: (1) The key of this kind of questions is to write the number of times students study according to the stem leaf diagram. The number of times students in group B go to the library is 7, 8, 9, 12. Then the average and variance can be obtained according to the formula; (2) Write all the data in (1), that is, the number of times of study. It is found that there are three people in group A and three people in group B, all of whom have studied for more than eight times. According to the classical probability problem, write the situation of choosing two people from six people, which is 15. Two students study in two libraries respectively, and there are five cases where the sum of study times is more than 20. See the test analysis for details.

Analysis of test questions: (1) When x=7, it can be seen from the stem leaf diagram that the number of times students in Group B go to the library is 7, 8, 9, 12.

What's the average? 3 points

The variance is 6 points.

(2) Remember that the three students in Group A are A 1, A 2, A 3, and the number of times they go to the library is 9 12,11; The four students in group B are B 1, B 2, B 3 and B 4, and the times of going to the library are 9, 8, 9 and 12 respectively. Choose two students who have studied for more than 8 times. There are 15 possible results, which are:

A 1 A 2，A 1 A 3，A 1 B 1，A 1 B 3，A 1 B 4，A 2 A 3，A 2 B 1，A 2 B 3，A3B65433。

If C is used to represent the event that "two selected students have just studied in two libraries, and the sum of study time is more than 20", there are five results in C, namely A 1 B 4, A2B-4, A 2 B 1, A 3 B 4,? 1 1 min

The two selected students just study in two libraries respectively, and the probability that the sum of study times is greater than 20 is 12.