There are 4 students in Group A, 3 students in Group B, and 2 students in Group C. The mean heights of the students in Group A, Group B, and Group C are 164 cm, 169 cm, and 170 cm, respectively. Find the mean height of all the students.
If the scores of 10 students in an examination are 87, 74, 66, 95, 83, 80, 90, 70, 56, and 75, find the standard deviation of these scores.