This problem seems confusing since different units are being used to represent concentration. The best way to solve this problem is via alligation, but in order to do so, I converted the mg/mL concentrations into % w/v so that everything has the same units. The 40 mg/mL injection is the equivalent to 4% w/v and the desired concentration of 13.5mg/mL is equivalent to 1.35% w/v. Once you get those values, just set up the alligation with the highest concentration on top (4%), the lowest concentration on the bottom (0.3%), and the desired concentration in the middle (1.35%). Once the alligation is set up, you should get an answer of 1.49mL of the 40mg/mL Tobrex injection is needed.
