A solution is prepared by diluting 200 mg of drug in 50 mL diluent to yield 4 mg/mL. How many mL are needed to deliver a 20 mg dose?

A solution is prepared by diluting 200 mg of drug in 50 mL diluent to yield 4 mg/mL. How many mL are needed to deliver a 20 mg dose?

• A. 3 mL
• B. 4 mL
• C. 5 mL
• D. 6 mL

Answer: (C)

Think in terms of concentration and volume. After dilution, the solution has 4 mg per milliliter (200 mg / 50 mL = 4 mg/mL). To obtain a 20 mg dose from this same solution, you need a volume such that the mg = concentration × volume: 20 mg ÷ 4 mg/mL = 5 mL. Checking the math: 5 mL × 4 mg/mL = 20 mg, which matches the desired dose. So, 5 mL of the diluted solution is required.

Think in terms of concentration and volume. After dilution, the solution has 4 mg per milliliter (200 mg / 50 mL = 4 mg/mL). To obtain a 20 mg dose from this same solution, you need a volume such that the mg = concentration × volume: 20 mg ÷ 4 mg/mL = 5 mL. Checking the math: 5 mL × 4 mg/mL = 20 mg, which matches the desired dose. So, 5 mL of the diluted solution is required.