There are 9 students participating in the rope skipping competition and 12 students participating in the long jump competition. How many students participate in both competitions

This is a simple set problem, the content of the first chapter of senior one

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1. There are 9 students participating in rope skipping competition and 12 students participating in long jump competition. How many students participate in both competitions at most?
2. Four students choose to apply for running, high jump and long jump, one for each. How many ways are there to apply? Why 81?
3. Four students choose to apply for three items of running, high jump and long jump, one for each. There are several ways to apply
4. The results of the three students in the long jump are: a 4 and 2 / 3 meters, B 4 and 15 / 7 meters, C 4 and 12 / 11 meters. Who is the first?
5. In a one minute rope skipping competition, the median score of 11 students is 52, and one student's score is 54. He () is in the top five A sure B possible C impossible
6. In a one minute rope skipping competition, the scores of 12 students are as follows: 122.119.115.110.108.105.102.100.98.97.96.88, the median is 103.5 The average is 105. Why is the average larger than the median
7. In the one minute rope skipping competition, the average number of Xiaohong's two jumps is 130. To make the average number of three jumps 135, how many times should she jump for the third time?
8. Find the rules! 1,3,8,19,42 (), ()
9. Find the rule 2, 3, 5? 8
10. Find the rules 1,2,2,5,3,8, [], []
11. There are 48 students in class 6 (1), accounting for 96% of the class. How many students are there in class 6 (1)?
12. Once again, in the rope skipping competition, class 31 has 20 people participating in the collective rope skipping competition and 10 people participating in the individual rope skipping competition. Among them, 6 people have participated in both the collective rope skipping competition and the individual rope skipping competition. How many people have class 31 actually participated in the rope skipping competition
13. The number of students in class 51 is between 30 and 60. If they are divided into a group of four, a group of six, or a group of eight, then there are several students in class 51
14. Students to participate in rope skipping competition according to a group of 4 or 8 or 10, group, just finished, to participate in rope skipping at least how many students?
15. In rope skipping competition, the number of people is between 100 and 150. If they are divided into five people, six people and eight people, they are just finished. How many people are there in rope skipping competition?
16. Students participate in rope skipping competition, according to each group of 6 people or each group of 8 people for grouping, can be divided into several groups, participate in rope skipping competition at least how many people?
17. The number of male students in class 61 is equal to that in class 62. The number of male students in class 61 accounts for 50% of the total number of students in the class, and the number of male students in class 62 accounts for 60% of the total number of students in the class
18. The number of students in class 5 (1) is about 40 to 60. The students in the whole class take part in rope skipping competitions and divide them into groups. Each group has 6 students or 8 students How many people are there in class five (1)?
19. There are 240 students in Grade 6, including 50 in class 6 (1), 60 in class 6 (2), 20.8% in class 6 (1), and the center angle of fan-shaped circle in class 6 (2) []
20. 9-1 = 8 * 1 25-9 = 8 * 2 49-25 = 8 * 3 81-49 = 8 * 4, what rule do you find by observing the above formula? Express this rule with formula (letter)