So in this problem we're trying to find the mass of calcium chloride that would contain 25 grams of chlorine. The key thing we have to remember here is that the percentages are going to be true for a sample, regardless of the sample size. So if I calculate the percentage of chlorine in any calcium chloride sample, it's going to be the same for all of them. And so I can actually do that, find the percentage of chlorine in calcium chloride by using our molar mass of the compound relative to the molar mass of the chlorine in that compound. So I'm going to first look at the molar mass of the whole compound, I have 1 calcium which has a mass of 40.01. And then I have chlorine, which has a mass of 35.45, and there are 2 of them. And when I add these values together, I end up with 110.91 grams per mol. Now I want to find the percent chlorine in my sample. And so I take the 2 times 35.45, or 70.90 divided by the total mass, 10, 110.91 and what that gives me is a percent of chlorine equal to 63.93%. So I know that's going to be true in all samples of Calcium Chloride including one that contains 25 grams of chlorine. So now I can say 63.93% chlorine equals the part over the whole. The part in this case is going to be the 25 grams, because our problem tells us that this is 25 grams of just the chlorine. So I know that's going to be in my numerator, and then I have x grams of CaCl2. And so I can divide by 100, and so I end up with 0.6393 equals 25 over x. And I can rearrange and solve this and find that x is going to be equal to 39.29 grams of calcium chloride. And we can actually easily go back and check our work, because we can take 25 divided by 39.29, and we should get an answer of approximately 63.93%.