You must also know the total weight of the beef. You forgot to mention it but let's say the beef weighs 450 grams and is known to contain 12 milligrams of calcium. Then the amount of calcium per 100 grams would be found like this: 12mg ... 450g, xmg ... 100g <=> x = 100*12/450 <=> x = 120/45 <=> x = 2.66mg
