Okay, let's continue our density discussion by doing this type of a problem, the density of a sphere. So it says the star is estimated to have a mass of 2.2 times 10 to the 36 kilograms. Assuming it to be a sphere of average radius of 7.6 times 10 to the 5th kilometers, calculate the average density of the star in units of grams per cubic centimeter. Okay, and I have all of the information that you were given from the problem, the mass of the star, the radius of the star, and the density of the star. Now remember, you want to solve the density of the star. Now remember, for the density, we need to know mass per unit volume. So in other words, we need to figure out what the volume is of a sphere, and how it actually relates to the radius. So if you recall from geometry class, volume equals 4, 3rd, pi, r, cubed. Okay, so r being the radius, right? So from this, we should be able to figure out the volume of the star. So 4, 3rd, times pi, times 3rd, 7.6, times 10 to the 5th kilometers, here. So let's solve for them. I have cubed times pi times 4 divided by 3. Okay, and if hopefully you get the same numbers I do, to two significant figures, right? So it's going to be 1.8 times 10 to the 18th kilometers. Okay, cubed. So when we look here, we want centimeters cubed. That's not kilometers cubed. We're going to have to, what do you say, convert from kilometers cubed to centimeters cubed. Also when we look over here, we've got kilograms with a mass of the star, but we want grams for the density. So let's remind ourselves how to change kilograms to grams. So 1 kilogram for every 1000 grams, kilograms cancel, and that's 1 to the third plus 36. So it's going to be 2 times 10 to the 39 grams. That's the mass of the star. Okay, pretty massive. So remember here, right? For every kilometer, we've got to figure out, well, what's the conversion? 1 kilometer. How many centimeters do we have per each kilometer? So remember, a meter is 1 to the, our kilometer is 1 to the third meters, and a centimeter is 1 to the second centimeter. So if you add those to every kilometer is 1 times 10 to the 5 centimeters. Okay, but we have cubed here, so we can multiply the times 10 to the 5 centimeters per kilometer, three times, or we could just do like this. So hopefully you see kilometers cubed gets canceled, but we've got to cube 1 to the fifth centimeters, or 1 times 10 to the fifth centimeters. When we do that, of course it's 5 plus 5 plus 5, which is 15, right? So it's 1.8 times 10 to the 18th times 10 to the 15th centimeters cubed, like that. Okay, so now let's multiply those two numbers together. So the times 1, e, 15, that way you get 1.8 times 10 to the 33rd centimeters cubed, because that's all the units we've got left. Okay, so we've got grams, we've got centimeters cubed, so let's figure out the density. Okay, density is grams per centimeters cubed, so 2 times 10 to the 39th, grams divided by 1.8 times 10 to the 33th centimeters cubed. So hopefully everybody sees we have the right units, then grams per centimeters cubed. And now we just plug and check. So 2, e, 3, 4, 5, 6. And we get... So this is two sick things, this is one sick thing, so there's only one sick thing. So 1 times 10 to the 1, 2, 3, 4, 5, 6. 6 grams per cubed, send. So that's the density of this particular star to the right number of sick things. Okay, so hopefully there aren't any questions if there are, feel free to email me about it. Okay, thank you.