.068443 mol/L
First we must find the molar mass of NaCl. We find the atomic mass of sodium and chlorine and add them:
Na: 22.9898
Cl: 35.453
22.9898 + 35.453 = 58.4428g/mol
The we find the number of mols contained in 400mg of NaCl:
.4g / 58.4428g/mol
.0068443 mol
Since 100mL is a tenth of a liter we can multiple the number of mols by ten:
.068443 mol/L