So we want to know what is the recommended daily amount of salt is if a cup of broth has 870 milligrams of salt and that os 30% of the recommended daily allowance. So we are looking for a number x that from which when we take 30% we get 870 milligrams. Lets turn that into an equation. (30/100)*x=870. So x=(870*100)/30=2900 milligrams. So the recommended daily allowance of salt is 2900 milligrams.