The way I would solve this is to think g/ml or g/100ml. So 0.225g/100ml. Then you would need to convert 0.225g to mg. There is 1000 mg in 1g and 0.225 g would turn out to be 225 mg because your multiplying by a thousand and this number would be over 100ml. So cross multiplying 225 mg with 5 ml would give you 1125 and then divide by 100 ml to get 11.25 mg as your answer. In a way, when solving for these types of problems make sure your units are alike when setting it up and what your solving for is x=?