A vial contains 50 mg per 5 mL. If 150 mg is required, how many milliliters are needed?

A vial contains 50 mg per 5 mL. If 150 mg is required, how many milliliters are needed?

• A. 5 mL
• B. 10 mL
• C. 15 mL
• D. 20 mL

Answer: (C)

The main idea is concentration and direct proportion. The vial has 50 mg in 5 mL, which is 10 mg per 1 mL. To get 150 mg, you need 150 mg ÷ 10 mg/mL = 15 mL. In other words, the volume scales with the dose when the concentration is fixed. If you used 5 mL, you’d get 50 mg; 10 mL would give 100 mg; 20 mL would give 200 mg. Therefore, 15 mL is required.

The main idea is concentration and direct proportion. The vial has 50 mg in 5 mL, which is 10 mg per 1 mL. To get 150 mg, you need 150 mg ÷ 10 mg/mL = 15 mL. In other words, the volume scales with the dose when the concentration is fixed. If you used 5 mL, you’d get 50 mg; 10 mL would give 100 mg; 20 mL would give 200 mg. Therefore, 15 mL is required.