 Okay. So, silver has two isotopes. One isotope contains 60 neutrons and has a percent abundance of 51.83, 9%. The other contains 62 neutrons, right? So let's write down what the problem gives us. So, silver and silver, we have these two isotopes, so thank you for seeing that. So, both of them are 47 for their atomic number, right? So, in order to find their mass number, right, we have to add that number of neutrons, okay? So, in this case, right, it's going to be the number of neutrons, right? Sorry. The mass number is going to be the number of protons plus the number of neutrons. And so we have 47 in every one and this one was 16 neutrons, right? So that's 107 and the mass number for this one is going to be the same protons plus neutrons, so 47 plus 62. So that's going to be 1, oh, so does everybody okay with that, can I raise that one? Okay. So now, we want to figure out what the relative abundance of each one of these things is. So, it says the one is 51.839 percent, okay? So this is relative abundance and then we'll say the mass is 107 and we know the mass down here is 109 and then the relative abundance, how do we figure this out? Say 100 minus 51.839, right here, 48.1 and 6.1 percent, okay? So, everybody's cool with doing all of that, right? Now, we want to figure out how do we get to the average atomic mass? So, the average atomic mass is going to be, you can do this two ways. You can turn this into a decimal right away if you want, so divide both of these by 100 percent, okay? Or you can divide it at the end by 100 percent, okay? So, let's just divide it by the end or divide it at the end by 100 percent because I think I have some examples already where I divide them at the beginning, okay? So, what do I do? I just take 51.839 percent times 107 and add that to 48.161 percent and divide that by 100 percent. Is everybody cool with that? So, again, it's the abundance of the isotope times the mass of the isotope plus the abundance of the isotope times the mass of the isotope all divided by 100 percent, okay? So, let's do this. So, again, you would expect it to be in between 109 and 107 and almost exactly in between due to the abundance of these two, right? Unfortunately, we can only go to three significant figures here. So, I'll write down what I have to five significant figures and then we'll cut it down to three. So, 107.963 AMU, I mean, that's what my calculator gives me because why? Because percentages are going to cancel but AMUs are not going to cancel so everybody see that too, okay? So, our units are percentage and then we've got to cut this down to three segments. So, everybody cool with that too? Why? Because we're multiplying here, this has to go to three segments. This has to go to three segments. This is an actual exact number so we don't have to worry about the segments that everybody's cool with that. So, when we take this to three segments, 108 AMU. So, that's the average atomic mass of silver given in this particular problem. So, everybody okay with this? An analysis of doing this type of stuff, okay? So, again, if you were given like four isotopes and maybe I'll do that later, one like that later, you just do the same thing. Abundance times mass plus abundance times mass and if you, there's another one, plus abundance times mass plus abundance times mass, as many isotopes as they can be. Is everybody okay with that? Divide it all by 100%. Okay, and that's how you get that average mass number that's written in red below the elemental symbol on the periodic table. Cool? Questions? Okay.