= Solution
For an ideal electrolyte with species number densities $n_i$, valences $z_i$, solvent permittivity $\epsilon$ and common temperature,
$$
\boxed{\lambda_D=\left(\frac{\epsilon k_BT}{e^2\sum_i n_iz_i^2}\right)^{1/2}.}
$$
Increasing salt concentration supplies more mobile charge to screen a surface perturbation, so a shorter distance is needed for the potential to decay. At fixed ion composition, permittivity and temperature, $\lambda_D\propto c^{-1/2}$.
A fivefold concentration increase therefore gives
$$
\boxed{\lambda_D'=\lambda_D/\sqrt5=0.447\lambda_D.}
$$
Equivalently, the inverse screening length grows by $\sqrt5$. Changing ion valence changes the ionic-strength weighting as well, and highly concentrated electrolytes can depart from this dilute-solution screening model.
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