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Units — Basic example

Watch Sal work through a basic Units problem.

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  • blobby green style avatar for user haylee dagnillo
    this example was easy to understand and easy to learn
    (11 votes)
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  • stelly green style avatar for user nikta
    Couldn't you just do 15*10 to get 150 and then choose the first one because if it is 3/4 then it will have to less than 150 and the only choice that is less than 150 is choice A
    (9 votes)
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  • piceratops seed style avatar for user Kimari
    why do we keep the units when we know what our final units are
    (4 votes)
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    • piceratops ultimate style avatar for user ritap1092
      Keeping the units is an easy way to see if you mix up the equation. For example, if you keep the units and end up with L/mg, you immediately know that you messed up, whereas without units, you might not catch that mistake.
      (2 votes)
  • starky sapling style avatar for user alokpatel
    its easy just divide 10.3divided by 4 and multiply it by 15 you will get 112.5 you don't need to do complex math
    (2 votes)
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  • leafers ultimate style avatar for user stpatrick749
    I multiplied 10 times 15 and then multiplied that by 0.75 and got the same answer. Is there any reason I shouldn't do it like that?
    (1 vote)
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    • piceratops ultimate style avatar for user Hecretary Bird
      No, there isn't. In these sort of converting-between-unit problems, all you're doing is multiplying, so the order doesn't matter. You can find a standard order of doing the problem that works best for you: Maybe it's in order of what's given in the problem, or maybe you start at what's being asked for and work backwards form there. Who knows. But as long as you have all of the conversion factors you need, any order works.
      (2 votes)
  • aqualine seed style avatar for user joulhayannanar
    Could I have used the calculator to do the calculation?
    (1 vote)
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  • blobby green style avatar for user Sam Williams
    So you convert the litters and milligrams?
    (0 votes)
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    • blobby green style avatar for user 1722541
      in a way yes i did it a different method so knowing that 1 liter = 10 mg , you'd just multiply 15 by 10 to get the amount in liter milligram or Lmg after that you multiply it by 3/4L causing you to just cancel the liters and making it milligrams :)
      (3 votes)
  • blobby green style avatar for user Sam Williams
    So you convert the litters and milligrams?
    (0 votes)
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  • blobby green style avatar for user elyce.johnson
    so do you combine the litres and the miligrams?
    (0 votes)
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    • mr pants pink style avatar for user Trek Session
      No, you don't combine the liters and milligrams; they are separate units. What Sal did was set up his equation so that liters were in the numerator and the denominator and so cancelled out. This was necessary because the question is asking for how many milligrams of nitrate the class would find in their water sample.
      (1 vote)
  • blobby green style avatar for user snishatschool
    i would reccomend putting the actual admin into the video
    (0 votes)
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Video transcript

- [Instructor] A high school class is measuring the amount of nitrate in a local stream. To be considered safe to drink, the maximum amount of nitrate that can be present in water is 10 milligrams per liter. The class takes a sample of 15 liters of water. If the number of milligrams per liter of nitrate in the stream water is 3/4 of the maximum that is safe to drink, how many milligrams of nitrate should the class expect to find in their sample? All right, this is interesting. So in this sample, in this 15-liter sample, they find that the number of milligrams per liter of nitrate is 3/4 of the maximum. Well, what's the maximum? Well, they tell us up here. They tell us, let me underline this. The maximum amount of nitrate that can be present in water is 10 milligrams per liter. So what the class finds is that they find that their sample has 3/4 of this maximum value. So what's 3/4 of 10 milligrams per liter? Let's just write that down. So the maximum is 10 milligrams per liter. In their sample, they find 3/4 of this maximum concentration. So let's just multiply that. We can multiply that times 3/4, which is going to be equal to what? That is 7 1/2 milligrams per liter. So that is 7.5 milligrams per liter. The way I think about this, 3/4 of a hundred is 75. So 3/4 of 10 is going to be 7 1/2. You could have done it other ways. You could say 10 times three is 30. 30 divided by four is 7 1/2, and you keep your units. So this is the concentration that they find in their sample, 7.5 milligrams per liter. And they do this, they find this concentration in 15 liters of water. So the total number of milligrams they find, well, you take the liters of water, 15 liters, and then multiply that times the concentration. 7.5 milligrams per liter. Now, the units should work out, and they do indeed. You have a liter being divided by a liter, so those cancel out. Then you're left with 15 times 7.5 milligrams. So we just need to find out what 15 times 7.5 is, so let's do that. So if you have 7.5 times 15, five times five is 25, five times seven is 35 plus two is 37, and then one times 75 is 75. So let's see, five plus zero is five, seven plus five is 12, and then four plus seven is 11, and you have one digit behind the decimal point. They would expect to find 112.5 milligrams, and there we go.