 Okay, so this next example is actually using two of the techniques that we've already talked about in class. The first one or more obvious one is density, but the other one is comparing two different substances and saying if their physical properties are the same, then we can kind of infer that they're the same substance. So this one is asking us if the new penny is made out of pure copper and it gives us the mass of the penny and the volume of the penny, which I've written there on the left side. So we need to remember the density formula, density equals mass divided by volume. And then we just plug those numbers in to the formula, so 2.49 grams divided by 0.349 cubic centimeters. And then when we calculate that out, we get 7.13 grams per cubic centimeter. So remember your significant digits, both of the numbers given have three sig figs, and put all that other stuff after the three that your calculator gives you. So the answer is 7.13 grams per cubic centimeter. So how can we see if this thing is made out of pure copper? Well, we look for the density of copper and there it is. So 8.96 grams per cubic centimeter and as you see, they're quite different. So we can say that this penny is not pure copper. Okay, let me know if you need any help with that one.