Define the data as the array
x = [300, 785, 670, 187, 760, 724].
The size of the array is n = 6
The standard deviation is
s = [Σ(x - μ)²/(n-1)]^(1/2)
The average is
μ = (300+785+670+187+760+724)/6 = 571
x-μ = [-271, 214, 99, -384, 189, 153]
(x-μ)²/5 = [14688, 9159, 1960, 29491, 7144, 4682]
Σ(x-μ)²/5 = 67125
s - √(67125) = 259.085
Answer: 259.1 (nearest tenth)