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## GMAT

### Course: GMAT > Unit 1

Lesson 1: Problem solving- GMAT: Math 1
- GMAT: Math 2
- GMAT: Math 3
- GMAT: Math 4
- GMAT: Math 5
- GMAT: Math 6
- GMAT: Math 7
- GMAT: Math 8
- GMAT: Math 9
- GMAT: Math 10
- GMAT: Math 11
- GMAT: Math 12
- GMAT: Math 13
- GMAT: Math 14
- GMAT: Math 15
- GMAT: Math 16
- GMAT: Math 17
- GMAT: Math 18
- GMAT: Math 19
- GMAT: Math 20
- GMAT: Math 21
- GMAT: Math 22
- GMAT: Math 23
- GMAT: Math 24
- GMAT: Math 25
- GMAT: Math 26
- GMAT: Math 27
- GMAT: Math 28
- GMAT: Math 29
- GMAT: Math 30
- GMAT: Math 31
- GMAT: Math 32
- GMAT: Math 33
- GMAT: Math 34
- GMAT: Math 35
- GMAT: Math 36
- GMAT: Math 37
- GMAT: Math 38
- GMAT: Math 39
- GMAT: Math 40
- GMAT: Math 41
- GMAT: Math 42
- GMAT: Math 43
- GMAT: Math 44
- GMAT: Math 45
- GMAT: Math 46
- GMAT: Math 47
- GMAT: Math 48
- GMAT: Math 49
- GMAT: Math 50
- GMAT: Math 51
- GMAT: Math 52
- GMAT: Math 53
- GMAT: Math 54

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# GMAT: Math 33

167-172, pgs. 174-175. Created by Sal Khan.

## Want to join the conversation?

- Hi!

Could you simplify 172 by first making it:

1>x^2

and then taking the square root from both sides:

1>x

Could you take the square root of the number one like that?

Many thanks!(5 votes)- That's basically what Sal does, except that he remembers to think about both the positive and negative square roots.(9 votes)

- #170 -- How did he figure out it's probably 4x when the worker ratio is increased by 2?(3 votes)
- You probably have gone on with your life just fine, without having this answered, but maybe this will help someone else.

So, why 4, allowing him to jump to the number of 20 for the managers?

Well, he noticed that if all you do is add 8 workers, the only thing that happens is to increase the denominator by 2, when the numerator remained the same (5), so only one-fourth of the increase showed up in the final ratio.

So the question is why?

From working with lots of these problems, he knows that the ratio they give us is a reduced fraction from the total.

So if the ratio is 5 managers to 72 workers, we could have any multiple of that as our actual numbers in the workforce.

To reduce to 72, with a numerator like 5, the denominator has to be a multiple of 72:

72, 144, 216, 288, 360, 432, 504...

So the actual workforce could be 5 managers to 72 workers or 10 managers to 144 workers or 15/216 or 20/288 or 25/360 or 30/432

After you add the 8 actual workers, only 1/4 of them showed up in the change of the denominator, but the relative number of managers did not change at all, so he jumped to the inspired idea that the amount the original fraction (likewise the final fraction) was simplified was a factor of 4.

That meant that he started with 20/288, and that changes to 20/(288 + 8) =

20/296 = simplifies to the new ratio of 5/74

So, during a test, an idea like this can save you time, but you might accidentally take away the wrong meaning. That is why if you do the problem by inspiration (intuition), you should probably make a quick reality check, such as adding the 8 to the number you got and making sure it still simplifies to the second ratio.

As Sal said, the safe way is to solve by algebra if you are not sure.(3 votes)

- You guys got any HOT POCKETS(1 vote)
- Hey guys just got home and got to a good job i am so now i’m going home and i am not home because she is a good friend to her kids and kids got her teacher kids and she had kids and she didn’t have to her teacher so they were kids and kids got to go home because they had a good time to get kids and then she got kids and she didn’t have a lot to worry kids kids were kids and she had a good time(0 votes)
- I don't understand it can someone help me?(0 votes)
- If y=4g then wath the value of 4g(0 votes)
- The value of 4g is y. There is no way to work out the true value of y without more information.(2 votes)

## Video transcript

We're on problem 167. If 1 plus 1 over x is equal to
2 minus 2 over x, then what does x equal? The first thing I would do is--
I don't like these x's in the denominator. So let's just multiply both
sides of this equation by x. So I'll get x plus-- x over x is
just 1-- equals 2x minus 2. Now I can subtract 1
from both sides. Actually, let's add 2 to both
sides, so you get x plus 3 is equal to 2x. Let's subtract x from
both sides. You get 3 is equal to x. And that's choice E. I think I have the hiccups. I hope you don't notice, or if
you do, I hope it doesn't bother you. Problem 168. Last year, for every 100 million
vehicles that traveled on a certain highway,
96 vehicles were involved in accidents. So 96 vehicles for every
100 million. So 100 times a million is 10
to the sixth vehicles. If 3 billion vehicles traveled
on the highway last year, how many of those vehicles were
involved in accidents? You could view this as the
fraction that will be involved in accidents, so we just
multiply that times the total number of vehicles. That's 3 times a billion. And they even write a billion
as 1 with 9 0's, so it's 3 times 10 to the ninth. So if we simplify this
a little bit, let's see what we can do. So 100 times 10 to the sixth,
this is the same thing as 1 times 10 to the eighth. You could view this as 10 to
the second times 10 to the sixth is 10 to the eighth. If you have a 10 to the ninth in
the denominator divided by 10 to the eighth, this cancels
out and you're just left with a 10. So the denominator becomes
1, and the numerator becomes 96 times 30. And 96 times 30 is-- let's put a
0 here and then 3 times 6 is 18, 3 times 9 is 27
plus 1 is 28. How many vehicles are involved
in accidents? 2,880, C. I think this number is low
compared to the reality. I think actually it's closer
to the 40,000 range. That's what want. How many of those vehicles are
involved in accidents? We multiply the fraction that
have an accident times the total number of vehicles, and
then you get your answer. 2,880. Next question, 169. 30% of the members of a swim
team have passed the lifesaving test.
So 30% passed. Among the members will have
passed the test, 12 have taken the preparatory course. So this the 70% not passed. And they say that 12 have taken
a course, so 12 course. And 30 have not taken
the course. How many members are
in the swim club? So many people have not
passed the test? They said that 12 have taken
a course, 30 haven't. I'm assuming that's
all of them. So the 70% that have not passed,
that is 12 plus 30 is equal to 42. And this is 70% of the total
people in the swim club. So 70% of the total
is equal to 42. So the total is equal to
42 divided by 0.7. 42 divided by 7 would be 6, but
since we're dividing it by 1/10 of that, it's going
to be equal to 60. And that make sense. 70% of 60 is 42. So there's 60 people in the
swim club, and that's A. Problem 170. In a certain company, the
ratio of the number of managers to the number
of production line workers is 5:72. If 8 additional production line
workers were to be hired, the ratio of the number of
managers to the number of production line workers
would be 5:74. So if we hire 8 more
workers, then the ratio is equal to 5:74. How many managers does
the company have? So this is interesting. Let's think of it this way. There's a quick way to think
about it, and then there's an algebraic way, and we'll
do both of them. But when we add 8 to the bottom
of this-- when we add 8 to the number of workers--
the ratio actually only increased by 2. So what's happening is we
probably have 4 times as many workers as this ratio
predicts. So the ratio is probably
20:288. And let's think about
that logic. Because 20/288-- this is 5/72 if
you were to divide the top and the bottom by 4-- and if you
were add 8 to the bottom, you would get 20/296. And then if you divide this by
4, if you just take this to the lowest reduced
form, you get 5. Divide that by 4. This divided by 4 is what? It's 74. So if you see that intuition
that I'm talking about, that when you add 8 workers it only
increased this reduced form by 2, that means that this reduced
form is probably 1/4 of the real number. Because 2 is 1/4 of 8. If that doesn't make sense to
you, don't worry about it. But if you did it this
way, you could say there must be 20 managers. And that would be a very fast
way of solving this problem. But we can also do it
alegebraically. So you could use this
information to say 5 times the workers is equal to 72
times the managers. And then you can use this
information to say 5 times the workers plus 40-- I'm just
cross-multiplying, so 5 times w8-- is equal to 72 times
the managers. And now we can do
some algebra. So let's subtract this from. Let me say minus 5w
is equal to minus 72 times the managers. Subtract this from this. These cancel out. You get 40 is equal
to 74 minus 72 is 2 times the managers. So then you get the managers are
equal to 20, which is what we figured out the first time. Maybe this is simpler for you. And I would probably do that
on the exam to make sure I didn't make a careless
mistake. Next question, 171. If x minus 1 squared is equal to
400, which of the following could be the value
of x minus 1? x minus 1 squared is equal to
400, so that means x minus 1 is equal to positive 20, the
positive square root of 400. And don't forget this. This also means that x minus 1
could equal minus 20, because minus 20 squared is also 400,
the negative square root. In this case, if you add 1
to both sides, you get x is equal to 21. In this case, if you add 1 to
both sides, you get x is equal to minus 19. So that's what the
values of x are. So what can be a value
of x minus 5? That's what they ask for,
they're not asking for the value of x. x minus 5 in this case would
be-- what's 21 minus 5? That's 16, minus 19 minus 5. That's minus 24. So x minus 5 could be 16,
or it could be minus 24. And they only have this one
there, so it's choice C. So they were just testing to see
if you remembered that x minus 1 could be the minus
square root of 400, the negative square root. Problem 172. Which of the following describes
all values of x for which 1 minus x squared is
greater than or equal to 0? So let's add x squared to both
sides of this equation. So then you get 1-- x squared
plus minus x squared, that zero-- is greater than or
equal to x squared. I'm just going to switch it
because that's just how I think. x squared is less
than or equal to 1. We're dealing with real numbers,
so x squared is going to be a positive number. So the only way that I can
square a positive number and have it be less than or equal 1,
is if x itself is less than or equal to 1. Or it could be greater than or
equal to negative 1, because those numbers will square
to being positive. So x would be less than
or equal to 1. But it can also be greater than
or equal to negative 1. And you could try it. Try negative 1/2. Negative 1/2 squared
is positive 1/4. No matter what, whether it's
positive or negative, this is going to be a positive number. And the magnitude can
be at most 1. So that takes us from
minus 1 to 1. And even 0 works. So let's see if that's
one of the choices. That's choice E. And I'll see you in
the next video.