We first calculate the amount, in moles, of NaCl that we will need.Amount of NaCl needed = 0.24 x 400/100 = 0.096mol.
Mass of NaCl needed = (23.0 + 35.5) x 0.096 = 5.616g
So to produce 400ml of 0.24M NaCl solution, accurately add 5.616 grams of NaCl to 400ml of deionised water.