Solution: the sum of squares of the scores of the first group of students = 20 * (s12+90 2).

=20*(6^2+90^2)= 162720

The sum of squares of students' scores in the second group = 20 * (S2 2+80 2)

=20(4^2+80^2)= 128320

The sum of the squares of the students' grades in the class =162720+128320 = 291040.

Class average =(80+90)/2=85.

The square of the standard deviation of the whole class S2 = 291040/40-852 = 51.

So the standard deviation of the whole class is s=√5 1.