Formula: standard deviation, s:
s = √ (s^2)
s^2 = ∑ (xi - x)^2 / (n - 1)
where
xi are the individual data
x is the median of the data
n is the number of data.
Then, first find the number of data: n = 7
Next, find the media: x = [56 + 78 + 124 + 34 + 67 + 91 + 20] / 7 = 470 / 7 = 67.1
Next, find ∑ (xi - x)^2
(56 - 67.1)^2 + (78 - 67.1)^2 + (124 - 67.1)^2 + (34 - 67.1)^2 + (67 - 67.1)^2 + (91 - 67.1)^2 + (20 - 67.1)^2
= 7,364.87
Finally: s^2 = [7364.87 / (7 - 1) ] = 1227.48 => s = √(1227.48) = 35.04
Answer: s = 35.04