Mathematics problem in the second volume of eighth grade ~ 1. There are 20 questions in the contest, one question 10, and 5 points will be deducted if one question is wrong or not answered. You must answer at least a few questions correctly to get 80 points. 1. If you want to answer at least x questions correctly, you should answer ***20-x questions incorrectly or incorrectly.

10x-5(20-x)>=80

15x & gt； = 180

x & gt= 12

Answer: Answer at least 12 questions correctly.

2. Number

3.( 1) Let the walking speed be x m/min. According to "leave home at 7 am and arrive at school between 7: 30 and 40", the distance traveled within 30 minutes is less than or equal to 2,400 meters, and the distance traveled within 40 minutes is greater than or equal to 2,400 meters;

(2) According to the listed inequality group, find out the small walking speed range. Solution: Solution: (1) Let the walking speed be x m/min. According to the meaning of the question:

{30x≤2400 40x≥2400，

(2) The inequality set of the solution (1) can be obtained: 60≤x≤80.

So Xiao Ming's walking range is 60 m/min -80 m/min.

So the answer is: {30x ≤ 2400 40x ≥ 2400; 60m/min-80m/min。