First draw out the equation. Mg + HCL = MgCl2 + H2Now balance this equation Mg + 2HCl = MgCl2 + H2Next we need to work out the moles of Mg. We know that we have 4g and Mg has a molar mass of 24. Mass/molar mass = moles. So 4/24 = 0.167 moles of Mg. In the equation we can see the ratio of Mg: HCl is 1:2. So we have 0.164 x 2 mol of Hcl = 0.333. To work out volume= mol/ conc. 0.333/0.2 = 1.665. The question asks for this in cm3 so the final answer is 1665 cm3.