$f'(x) = \dfrac{e^{-x}}{(1+e^{-x})^2}$ $----(1)$
We have to find the derivative value at $x$ such that $f(x)= 0.4$
$\dfrac{1}{1+e^{-x}} = 0.4$
$1+ e^{-x} = \dfrac{1}{0.4} = 2.5$
$e^{-x} = 2.5 - 1 = 1.5 $
Substitute $e^{-x}$ value in $(1)$ then it will be,
$f'(x) = \dfrac{1.5}{(1+1.5)^2}$
$=0.24$
So the answer is $0.24$.