(1) The data of physical exercise time of 65,438+00 students in Grade Two of A and B schools in that month are as follows:

Schools A: 3, 5, 10,1,13, 15, 17, 18, 21;

School B: 3, 3, 4, 4, 5, 1 1, 12, 14, 18, 36.

Therefore, the mode, median and average of physical exercise time of students in the two schools in that month are as follows:

(2) Suppose that the average monthly physical exercise time of students in a school is. x = 1 10(3+5+ 10+ 1 1+ 13+ 15+ 17)。

Then S2 =110 [(3-13) 2+(5-13) 2+(10-/3) 2+. 2 +( 13- 13 ) 2 +( 15- 13) 2 +( 17- 13) 2 +( 17- 13) 2 +( 18- 13) 2 +(2 1- 13) 2 ]

= 1 10 ( 1 0 2 + 8 2 + 3 2 + 2 2 + 2 2 + 4 2 + 4 2 + 5 2 + 8 2 ) = 1 10 ×302=30.2 .

(3) According to the stem-leaf diagram, the number of students who are seriously inadequate in physical exercise in two schools, A and B, is 2 and 5 respectively.

Then the frequency of students' serious lack of physical exercise in schools A and B is:

The frequency of school A is 2 10 =0.2.

The frequency of school B is 5 10 = 0.5.