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### Course: Digital SAT Math > Unit 7

Lesson 2: Unit conversion: medium# Units — Basic example

Watch Sal work through a basic Units problem.

## Want to join the conversation?

- cant I just know there is 150mg in 15l so 3/4 * 150 = 112.5mg

we Asians got our own trick

I'm from Nepal(68 votes) - 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(27 votes)
- There are more than one way to solve a problem. As long as the answer is right and you understand what you're doing, you should be fine.(13 votes)

- this example was easy to understand and easy to learn(15 votes)
- Why does 7.5 mg/L multiply 15L? I don't understand that part.(7 votes)
- You have 15 liters. 3/4 of 10 mg/L is 7.5 mg/L (milligrams per liter).

In other words, for every liter you have 7.5 milligrams of nitrite, so multiply 7.5 by 15 to find how many milligrams of nitrite are found in 15 liters.

7.5 mg/L * 15L = 112.5 mg(9 votes)

- 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(8 votes)
- That's what I did lol(2 votes)

- he forgot to say test takers you got this(5 votes)
- why do we keep the units when we know what our final units are(4 votes)
- 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.(3 votes)

- anyone can explain me this example ??(2 votes)
- The maximum amount is 10 mg = 1L. They have x mg = 15L. I just multiplied 15x10, so the final answer was 150 mg = 15 L. However, their amount is only 3/4 of the amount, so you can already know the answer by elimination, but if you want to make sure you're right, you can calculate 150 x 3/4. 150x3 is 450 and 450/4 is 112...(7 votes)

- how do I divide a whole number by a decimal like 1.6(3 votes)
- A pool can be filled by 3 pipes in 4 hours. If one of the pipes is turned off after 2 hours, how long will it take the other two pipes to fill the pool? Solve this, and explain things step by step(3 votes)
- in addition to 2 hours, it will take 3 hours therefore a total of 5 hours(1 vote)

## 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.