In addition, the number of students who failed the math test in senior three in a sample is 5.
∴ It is estimated that the passing rate of mathematics scores in the joint entrance examination of senior one and senior three in a school is1-530 = 56;
(2) The average grades of Grade A and Grade B of senior three students in the sample are a 1 and a2, respectively.
According to the stem leaf diagram,
30(a 1-a2？ )=(7-5)+55+(2-8)+(5-0)+(5-6)+…+92= 15,
∴a 1-a2= 1530=0.5.
∴ Use samples to estimate population, so estimate x 1-x2? The value of is 0.5.