Mols of acid needed: 0.4 liters x 0.10 mol/liter = 0.04 mol
Volumen of acid in the bottle than contaiins 0.04 mol of acid: V = #moles/M = 0.04 mol/12.1 mol/liter = 0.003306 liters = 3.306 ml
Then you need 3.3 ml of acid from the bottle and 396.7 ml of pure water to get 400 ml of acid 0,1 M
