If the patient weight is 250 lb and the doses are 0.8mg/kg/hour for 12 hours, then the total amount of theophylline needed would be:
250lb * 0.453592kg/lb* 0.8mg/kg/hr * 12hr= 1088.6208mg
If the solution contains 800mg of theophylline for 100ml volume, then the volume needed would be: 1088.62 mg / (800mg/100ml)= 136.07ml