You have three tubes containing solutions of the protein human chorionic gonadotrophin (HcG) in PBS. All three contain 2 ml. Tube A is at a concentration of 28 mg/ml. Tube B is at a concentration of 78 mg/ml. Tube C is at a concentration of 40 mg/ml.

If you want to make as much 10 mg/ml HcG as you possibly can, how much PBS do you need to add to the combined tubes?