 So, percents are the number of items with a particular characteristic out of a total number of 100. So say you have 55% of people with brown hair in a room. So that just means that 55 out of 100 people in the room have brown hair. So remember that from working with percentages we talked about that it's easy to go from a percent to a fraction because it's the number out of 100. So we're going to use that to look at some laboratory questions to kind of think about how we might prepare certain solutions that the word problem has a percentage in it. So if a solution is made up of just water and ethylene glycol and you need to prepare a 45% ethylene glycol solution and you need 450 mLs, how would you do that? So we can say we need 45 out of 100 parts to be ethylene glycol and we need 450 mLs. And so then we're going to solve this proportion, 45 times 450 equals 100x, divide both sides by 100. That's going to leave you with x equal to 202.5 mLs of ethylene glycol. But this is not the end of the problem. You need to know how to prepare the solution. So you know that 45% of the 450 mLs is ethylene glycol. So that's 202 mLs of ethylene glycol, but the rest is water. So to get to 450, you're going to say that you need to add 202.5 mLs of ethylene glycol and bring to volume with 247.5 mLs of water. So you always need to remember when you're asked how to prepare a solution that you're not just telling me how much of the solution is ethylene glycol, but how to prepare that solution. So how do you get 450 mLs? Because if you just put 202.5 mLs of ethylene glycol into a container, that's not a 45% solution. And so you'll need to bring it to volume using 247.5 mLs of water. And the way that we got that, just so that it's really clear, is that 450 minus 202.5 leaves us with 247.5. So we know that together this 202.5 mLs of ethylene glycol and the 247.5 mLs of water will give you your 450 mLs of a 45% ethylene glycol solution. So it's a little bit different if you're preparing a solution with a solid or something that you have to weigh out. And so this question asks how you would prepare 500 mL of a solution that is 20% weight to volume. And so your clue, I'm sorry, that you need to weigh something out is here in this weight to volume. So you'll know that you're dealing with a solid and not necessarily a liquid like we were in the last problem. And so you can really approach this one of two ways. We can do it the same way we did our percentages, where you have 20 out of 100 is equal to how much of 500 mLs. So then you've got 10,000 equals 100x. And you divide both sides by 100. And you'll have 100 grams is the amount that you need to add to your solution. Alternatively, if it makes more sense for you to use a decimal, we know that 20%, if we get rid of our decimal sign and move, or get rid of our percentage sign and move the decimal is 0.2, 0.2 times 500 is equal to 100 grams. So either way, you can figure out that you need to add 100 grams. Where it's a little bit different is that you can't just dissolve 100 grams into 500 milliliters and call it a day. Because that mass of whatever you're adding to your solution has some volume. And so if you add that to 500 mLs, you're going to end up with a solution that is actually larger than, greater than 500 mLs. And so what you want to do is then dissolve 100 grams in a partial volume. So maybe you take 250 mLs, dissolve that 100 grams in 250 mLs, and then bring to volume equal to 500 mLs. So then you continue to add water until you hit 500 mLs. And you'll have a solution that's 500 mLs of a 20% weight to volume. So we're going to do one more, a little bit more complicated problem that involves percentages. And it has two steps. This is the first step. So there are approximately three times 10 to the ninth base pairs in the human genome. Chromosome one is the largest of your chromosomes and contains about 8% of your genome. So what is the approximate number of base pairs that are on chromosome one? So we have three times 10 to the ninth in the total genome base pairs. So we're just going to call those BP. And chromosome one has about 8%. And I'm just going to do this by creating a decimal out of our percentage. And so times 0.08. And that should give us the number of base pairs on chromosome one. So I have 2.4 times 10 to the eighth. So don't let it worry you when you start seeing things like base pairs and really large numbers. You're still going to approach this the same way that you approached that last one. You can just multiply the percentage in form of a decimal by your original number and you can determine how many base pairs on chromosome one. But I'm going to take it a step further and say that if genome sequencing is 99.8% correct, what is the number of base pairs on chromosome one that are incorrect? So we're going to have that 2.4 times 10 to the eighth base pairs. If we have 99.8% correct, we have 2.2% incorrect, right? And so since the question asks how many are incorrect, we first have to convert our percentage to incorrect chromosomes. I'm sorry, incorrect sequencing of base pairs. So if we get rid of our percentage sign and move this decimal point to spaces, we have 0.002 that are incorrect. And if you multiply that out, you have 4.8 times 10 to the fifth incorrect base pairs. So again, this seems very confusing and students might look at this and get scared off a little bit because it's a lot of percentages, it's a lot of math, it's a lot of scientific words that they may or may not be familiar with, but really if you just break it down, you can see that it's just multiplying using percentages as decimal points and determining the amount of something that is that percentage.