yo, jc - i'm in your class. you don't have a PM button, so i'm not sure how to help you, but it's just a matter of converting units. we're being asked how much of the stock solution must be added to yield 2000pg/mL. the volume of the stock solution is so small compared to that of the diluent that you can ignore that part, and just worry about getting the right mass of IFN-gamma into the 800uL.
convert the IFN-gamma stock solution of 65 ng/mL to pg/mL. then do a ratio: 2000pg/mL = ?? pg/.8mL. solve to get: ?? = 1600 pg of IFN-gamma. divide by the stock concentration: 1600pg / (65000pg/mL) = .0246mL of stock solution = 24.6uL.
note: u = mu
