A solution contains `75 mg NaCl` per `mL`. To what extent must it be diluted to give a solution of concentration `15 mg NaCl` per `mL` of solution.

Explanation

To solve the problem of how much to dilute a solution containing 75 mg of NaCl per mL to achieve a concentration of 15 mg of NaCl per mL, we can use the concept of dilution, which is based on the principle of conservation of mass.

### Step-by-Step Solution:

1. **Identify Initial and Final Concentrations**:
- Initial concentration (C) = 75 mg/mL
- Final concentration (C') = 15 mg/mL

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