Answer
The $Na^+$ concentration is equal to 55 mEq/L
Work Step by Step
In order to balance the charges, the sum of the concentrations (in equivalents per liter) of the negative ions must be equal to the same of the positive ions.
$$C_{Cl^-} + C_{HPO{_4}^{-2}} = C_{Na^+}$$ $$40. \space mEq/L + 15 \space mEq/L = C_{Na^+}$$ $$55 \space mEq/L = C_{Na^+}$$