= Solution
At a common membrane <voltage>, an <ion> is in electrochemical equilibrium only if its own <Nernst potential> equals that <voltage>. The four independently calculated equilibrium voltages are
$$
\boxed{E_K=-97.74\ \mathrm{mV},\quad E_{\rm Na}=+66.60\ \mathrm{mV},\quad
E_{\rm Ca}=+130\ \mathrm{mV},\quad E_{\rm Cl}=-90\ \mathrm{mV}.}
$$
Thus at $V_m=-90$ mV only chloride is at its listed equilibrium <voltage>. <Potassium> has an outward positive-current driving <voltage> $V_m-E_K=+7.74$ mV. <Sodium> and <calcium> have negative driving voltages, $-156.60$ and $-220$ mV respectively, favoring inward positive-ion movement when their channels are open. Chloride has zero net electrochemical drive under these ideal conditions.
A resting potential is not a claim that every permeant <chemical species> is in <thermodynamic equilibrium>. A membrane with several <electrical conductances> can have zero \b[total] <electric current> even though individual ionic <electric currents> flow and cancel. Pumps maintain the <concentration> differences by consuming <Gibbs free energy>. Conversely, a closed channel can prevent substantial <ion> flux even when the <ion>'s <electrochemical potential> difference is nonzero: absence of flux due to impermeability is not the same as electrochemical equilibrium. The listed <concentrations> could not all be passive equilibrium distributions at one <voltage> because their <Nernst potentials> differ.
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