Question

Calculate the mass of solvent that would contain 3.0 mg of a drug if the concentration was 3.5%

Answer 1

3.5% what? w/w, w/v, v/v. I assume w/w from the wording of the problem.
3.5% w/w means 3.5g/100 g soln which is 3500 mg/100 g soln. You want 3.0 mg so you want
100 x 3.5/3500 = ? g soln.