A vial of streptomycin sulfate is to be reconstituted for IM injection in a hospitalized patient. The vial contains 5 g of the medication. The
instructions say to add 9 mL of sterile water; this results in a
concentration of 400 mg/mL. What is the volume of the dose in mL
needed to fill a doctor's order for 500 mg daily?
1.25 mg
1.25 L
1.25 mL
1.25 mg/ml

Explanation:

9 mL contains 400 mg/ mL

If you want a dose of 500 mg you would need to take 1.25 mL of the dilute solution.

This problem couldn't be more deceptively worded. The only fact you can rely on is the concentration becoming 400 mg / mL

You come to the answer by figuring out how to get 500 mg.

400 mg / mL * x = 500 mg      Divide by 400 mg / mL. Watch the units.

x = 500 mg// 400 mg / mL      The mg cancel out.

x = 1.25 mL

it should be