 Okay, so let's do the same half-life problem again. The half-life of cobalt-60 is 5.3 years. How much of a 1.00 meg sample of cobalt-60 is left after a 10-year period. Okay, this time we're actually going to do it quick and dirty way, so much faster. Remember, what are we looking for? We're still looking for the mass at time t, okay? But in this case, we're just going to use the fact that we know the half-life already, okay? So the first thing we're going to have to do is figure out, well, how many half-lives have passed from 0 times 0 to time t, okay? So to figure that out, so the number of half-lives that have passed, that's going to be t divided by t half, okay? So when we do that, what do we got? 10.6 years divided by 5.3 years, like that. Years cancel and we get, well, 2.0, if you will. So two times, two half-lives have passed. Is everybody okay with that sort of logic? Two half-lives have passed, okay? So after one half-life, remember, what does a half-life mean? Half of the sample is gone after one half-life, okay? So after, so the mass at t one half, so half of the original is going to be, well, if we start out with one milligram, what's half of that? 25. Yeah, 0.5 milligrams, so 0.500 megs specifically, in this case of cobalt 60. So that's after one half-life, but how many half-lives have we got? Two, right? So the mass after two half-lives is going to be, well, after one half-life, it's 0.5, so what's going to be after two half-lives? Yep, 0.250 megs, the answer. So much easier than doing it the other way, but the other way is very general, okay? Any questions on this one? Okay, wonderful.