(1) The total number of third-grade students in a school is n. 。

From the meaning of the question = 0.05, the solution n = 600.

In the sample, the number of students in grade one and grade three of a school who failed in mathematics was 5. Based on this, it is estimated that the passing rate of mathematics scores in grade three of a school is 1-=.

(2) Let the average samples of two proofreaders A and B be'1and' 2' respectively.

According to the stem leaf diagram, 30 ('1-'2) = 30'1-30' 2.

=(7-5)+(55+8- 14)+(24- 12-65)+(26-24-79)+(22-20)+92

=2+49-53-77+2+92= 15.

So' 1-'2 = 0.5. Therefore, the estimated value of 1-2 is 0.5.