The health-care provider prescribes an intermittent infusion of nafcillin (Unipen) 800 mg IV for a pediatric client. The drug is dispensed in a concentration of 250 mg/ml, and the recommended infusion concentration is 40 mg/ml. When adding the prescribed dose to the volume control device, how many ml of IV solution should the nurse add to dilute? (Enter numeric value only. If rounding is required, round to the nearest tenth.)