= Solution
In the planar <Debye–Hückel approximation>, a surface potential decays as $\psi(x)=\psi_s e^{-\kappa x}$, with $\kappa\propto\sqrt I$ at fixed <temperature> and solvent <permittivity>; $I$ is the <ionic strength>. For a monovalent salt, reducing its <concentration> by ten reduces $\kappa$ by $\sqrt{10}$. Calibrating the first decay from the supplied potential ratio, and assuming unchanged $\psi_s$, gives
$$
\boxed{\psi_{\rm new}(5\ \mathrm{nm})=35\left(\frac{6.6}{35}\right)^{1/\sqrt{10}}\ \mathrm{meV}=20.7\ \mathrm{meV}.}
$$
Here the printed meV unit is interpreted as the electrostatic potential energy of a singly charged probe; the same ratio calculation applies to potentials in mV.
This numerical answer uses an effective exponential decay calibrated from the data. The inferred initial <Debye length> is $5/\log(35/6.6)=3.00\ \mathrm{nm}$, whereas the standard dilute aqueous-electrolyte expression at room temperature gives about $0.96\ \mathrm{nm}$ for $0.1\ \mathrm M$ monovalent salt. The numbers therefore are not consistent with that absolute ideal-electrolyte model. Moreover a spherical colloid has an additional radius-dependent geometric factor, and its surface potential need not remain constant when salt is changed. Without a surface boundary condition and geometry, the new potential is not uniquely specified beyond the stated planar, fixed-potential approximation.
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