How do you find the answer 26% of x= 45.5?

Since each bottle is made up of 2-liter of the active solution, in order to determine the number of doses of the medication that can be made, we just have to divide 2 L by 4x10^-3.
                  number of doses = (2 L) / (4x10^-3L/dose) = 500 doses
Therefore, the pharmacy can make 500 doses of the medication.