The equilibrium potential of a biological membrane for $Na^+$ is $55$ mV at $37^\circ$ C. Concentration of $Na^+$ inside the cell is $20$ mM. Assuming the membrane is permeable to $Na^+$ only, the $Na^+$ concentration outside the membrane will be __________mM.

(Faraday constant: $23062 \text{ cal} \cdot V^{-1}\cdot \text{mol}^{-1}$, Gas constant: $1.98 \text{ cal.mol}^{-1}.K^{-1}$)