Let "x" be the grams needed to prepare a 20 mg/mL mass concentration solution, let "V" be the volume of the solution, in this case 32 mL, therefore the mass concentration will be equal to:
mass concentration = x/V
Clearing x, we have:
x = mass concentration * V = (20 mg/mL) * (32 mL) = 640 mg
one gram is one thousand milligrams, therefore, we have: