First you find how many pounds for 1 litre of blood. Trick is if you divide x by y, the answer you get will follow the unit of x. So if you divide 150 pounds by 4.5 litres, the answer will be 33.33 pounds per litre. So how many 33.33... is there in 250? Just divide 250 by 33.333 or (150/4.5), you'd get 7.5. That means for there's 7.5 parts of 33.333 pounds in 250 pounds. And if 33.333 pounds is equal to 1 litre of blood, you get 7.5 liter!
