This is a question in very starting chemistry. If this was a problem, if you needed a formula, or needed the indications of a teacher, if you were stuck by that question, and you are starting chemistry, let me suggest you do yourself a big favour:
Revise SIMPLE PROPORTIONS!
Whether it needs ten minutes and then it comes back, or whether you take a school arithmetic book and work through numbers of examples, do it till it is easy and obvious.
Because ALL the quantitative problems you will meet and be asked to solve for the first years in Chenistry will be really about nothing but proportions, all the quantitative experiments for which you will have to calculate some answers will be about proportions, the very laws of Chemistry are about proportions (and Dalton's Laws or Gay-Lussac's Law are actually called Laws of Proportions - atomic theory was based on them). Indispensible concecpts like mole and atomic and molecular masses are about proportions or ratios. And even when later they are also about something else, proportions will still be used as an essential part of almost everything, in theory, examinations, or laboratory practice.
And many students have a hard time with them. They account for about the second most frequent kind of question on this subforum. Why? Why are they stuck like you? I guess because they never learned proportions at school, but more likely they learned way back in school but have forgotten, or learned but that was called arithmetic and this is called chemistry and students don't carry stuff over between subjects. And teachers of Chemistry don't reckon to teach arithmetic all over again, they assume the students do or should know it and hence that their uses of proportions are obvious.
So if you get the idea of proportions or ratios right and familiar you should almost sail through the quantitative part of chemistry, otherwise you'll stumble and be confused or forget stuff and maybe get to hate it. So I suggest you do yourself that ^^ favor.
