Since the ordered dose is 15 mL or 15cc and what you have on hand is 320mg/5ml, in order to figure out how many mg are in 15 ml you need to multiply the 320mg by 3 to get your answer. You would multiply by 3 because since you have to multiply the 5mL by 3 to get 15mL you also need to do the same for the mg. Ie: 320mg/5ml 15ml/5ml = 3 ml; multiply 320mg x 3 = 960mg in 15 ml.