You weigh a gallon of 2% milk in science class and learn that it is approximately 8.4 pounds. you pass the milk to the next group, and then realize that you teacher wants it in kilograms, not pound. explain how you can adjust your answer without weighing the milk again. then give you the weight in kilograms.

Well first of all the kilogram is a unit of mass not a unit of weight. Your teacher should never have asked for weight in kilograms. If your class is meeting on Earth then one kilogram of mass weighs about 2.2 pounds. You can just divide your 8.4 pounds by 2.2. The quotient is 3.9 and that's the mass of the milk in kilograms. Be careful. If your class is meeting on some other planet then one kilogram of mass doesn't weigh 2.2 pounds there.