You have a stock bottle of HCl (aq) which is 12.1M, You need 400 mL of 0.10M HCl. How much of the acid and how much water will you need?

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