The concentration of the vial is 0.5 %. We will suppose that it is % w/v. The concentrarion % w/v is defined as:
% w/v = mass of solute in g/volume of solution in mL * 100
We have to find the mL that we have to administer to deliver 20 mg. Let's convert those mg into g. We know that there are 1000 mg in 1 g.
1000 mg = 1 g
mass of solute in g = 20 mg * 1 g/(1000 mg)
mass of solute in g = 0.020 g
Now that we know the concentration of the solution and the mass in grams of the medication, we can find the volume in mL.
% w/v = mass of solute in g/volume of solution in mL * 100
0.5 = 0.020 / volume of solution in mL * 100
volume of solution in mL = 0.5/(100 * 0.020)
volume of solution in mL = 0.25 mL
Answer: we need to administer 0.25 ml