In order to enhance the awareness of environmental protection, on the day of World Environment Day on June 5, under the guidance of the staff of the Environmental Protection Bureau, the study group of "Controlling Noise Pollution" composed of several "environmental protection guards" conducted a sampling survey on the noise level (unit: dB) of 40 noise measuring points in the city at a certain moment, and processed the survey data (assuming the measured data is a positive integer). The frequency distribution table is as follows:

Answer the following questions according to the information provided in the table:

A= in (1) frequency distribution table

eight

，b=

12

，c=

0.3

(2) Complement the complete frequency distribution histogram;

(3) If there are 200 measuring points in * * * of the whole city, how many measuring points with noise level less than 75dB at this time?

Through analysis, it is found that the frequency of (1) in a problem is directly proportional to the frequency, so the values of a, b and c can be obtained relatively simply.

(2) In addition, the height of the rectangle in the frequency distribution histogram is proportional to the frequency, that is, the number of measurement points, so it is easy to determine the height of each rectangle;

(3) estimate the population with the sample, and the frequency of the measuring points with noise sound level less than 75dB in the sample is 0.3, which can be solved by multiplying the total number.

(1) According to the proportional relationship between frequency and frequency, we can know that 40 1a02bc, first we can get a=8, and then we can get b= 12 by 40-4-6-8- 10= 12.

(2) As shown:

(3) The frequencies of measuring points with noise level less than 75dB in the sample are calculated as 0.3, 0.3×200=60,

At this time, there are about 60 measuring points with noise level less than 75dB.

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