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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 10

55-60, pg. 159. Created by Sal Khan.

## Want to join the conversation?

- Sometimes, I use to do the same mistakes that Sal does, but a very interesting thing is how he realize every time he does a mistake almost immediately.

It would be very helpful to analyze what mental techniques he uses ?(16 votes) - The easiest way to solve Q57, is to look at the denominator. So 15g in 100cm3, equals to .15g/1cm3. So for 45cm3, you just times .15*45=6.75(4 votes)
- On Question 57, He says something like, "Which is not a 'fermat prime'"? What is a 'fermat prime'?(3 votes)
- Fermat primes are prime numbers that can be written in the form 2^k + 1, where k is an integer and a power of 2.(1 vote)

- Question 60: Is angle QPS = x?(2 votes)
- At3:03, why is 2^1 a "power of 2"? He says it's "2^0".(3 votes)
- wow i make the same mistake(2 votes)
- how does this relate to the x variable?(1 vote)

## Video transcript

We're on problem 55. If y is equal to 4 plus x minus
3 squared, then y is lowest when x is equal to-- so
think about when will this expression be the lowest
possible number? Well, if you think about this
whole term, right here, this x minus 3 squared. Any number squared, if we're
dealing with real numbers, is going to be a positive
number, right? So you could say that x minus
3 squared is going to be greater than or equal
to 0, right? 0 is kind of its
minimum point. So this whole value is going
to be as small as possible when x minus 3 squared is as
small as possible, right? Because that's the variable
part of it. And this is going to be as small
as possible when x minus 3 is equal to 0. And when is x minus 3 going
to be equal to 0? We could write it out. When is x minus 3 equal to 0? Well, add 3 to both sides. It's when x is equal to 3. So y is lowest when x minus
3 is equal to 0 or x is equal to 3. And that's choice D. And all you have to recognize is
the lowest value that this function can attain is when this
term goes to 0, because otherwise it's a
positive term. It will only add to the 4. Next problem. 56. Which of the following
is not equal to the square of an integer? So they say the square root
of the square root of 1. The square root of 1 is 1. And the square root of that
is going to be equal to 1. You could have said negative
1, but then that would have been undefined. Fair enough. One of the roots of
the roots is 1. And this is the square
of an integer, right? This is equal to 1 squared. 2. Square root of 4. Square root of 4,
that equals 2. Now is that the square
of an integer? No. That's equal to square root of 2
squared, but this clearly is not an integer. Right? So the choice is 2. 2 is not equal to the square
of an integer. We could look at
the other ones. 18 divided by 2. Yes. 9 is the square of 3. 41 minus 25. That's 16. 36. So all of the other ones are
squares of integers. That's a fairly straightforward problem, I think. We're on problem 57. Fermat primes are prime numbers
that can be written in the form 2 to the K plus 1,
where K is an integer and a power of 2. Which of the following is
not a Fermat prime? So A, 3. Well 3 is equal to what? It's equal to 2 to
the 1 plus 1. Right? And in this case K is 1,
which is a power of 2. That's 2 to the 0. B is 5. Well, 5 is what? That equals 2 squared plus 1. 4 plus 1. And this is definitely
a power of 2. 2 is a power of 2. 2 to the 1st. C. 17. That's 2 to the 4th plus 1. Right? 2 to the 4th is 16 plus
1 and 4 is 2 squared. So that's a power of 2. Choice D. 31. The closest power, if we
subtract 1 from it. 2 to something does not
equal 30, right? That would have to be true
if 2 to the K plus 1 is equal to 31. Then 2 to the K would
have to equal 30. And I can't think of an integer
where I take 2 to some power and get 30. So the choice is D. D is not a Fermat prime. And if you wanted to make
sure that it's not E. E is 257. Well that's just 2 to
the 8th plus 1. 2 to the 8th is 256. And that's 257. So the answer is D. 58. If x squared is equal to 2y to
the third, and 2y is equal to 4, they want to know what x
squared plus y is equal to. OK, x squared plus y. Well, they tell us
that x squared is equal to y to the third. So let's just substitute that. Right? So then we get 2y to the third
plus y, using this information and that information. And they tell us 2y
is equal to 4. Well, just divide both sides by
2, you get y is equal to 2. So let's substitute
that into this. What's 2 to the third power? Well, that's 8 plus 2. So I don't want to divide. So that equals 16 plus 2,
which is equal to 18. Which is choice E. Next problem. 59. A glucose solution contains 15
grams of glucose per 100 cubic centimeters of solution. If 45 cubic centimeters of the
solution were poured into an empty container, how many grams
of glucose would be in the container? So we can just set up a ratio. We have 15 grams per 100, so how
many grams are we going to have per 45 centimeters cubed? How many grams, right? So they still want to
know grams. All the units are the same. And so let's see if we
can cross multiply. We could say 15 times 45 is
equal to 100 times x. So 15 times 45 is
equal to 100x. And just to make the
math easy, let's divide by some numbers. Let's divide both sides by 5. So then this becomes 20
and this becomes 9. We could divide both sides by 5
again and then this becomes 4 and this becomes 3. And now the numbers are
easy to deal with. 3 times 9 is 27 is
equal to 4x. Divide both sides by 4. You get x is equal to 27/4. 27/4 and 4 goes into 27. what is that? That is equal to and 6 and 3/4,
and all their numbers are in decimals, so that's 6.75. Which is E. Right? 4 times 6 is 24, and you
have 3 left over. Right. Next problem. Problem 60. In the figure of P, Q, R,
S is a parallelogram. Let me draw this. This looks interesting. So I have to draw a
couple of lines. That's one line. And they have another
line that looks something like that. It's a parallelogram. Let me connect the lines. It looks like that. Let's see, this line actually
keeps going. Let me draw that in. They tell us, this is P. This is Q. This is R. And this is S. And they tell us that
this is 140 degrees. They tell us that this is 2y. And they tell us
that this is x. And then they say, in
the figure above, what is y minus x? So since we know this is a
parallelogram, we know that this line is parallel to this
line, and we know that this line is parallel to that line. That actually tells us a lot of
information, especially if we continue the lines over. Then everything we learned in
geometry about parallel lines starts to hold. Let me draw it in a
different color. So if this line, in green, is
parallel to this line, then you can view this line right
here as a bit of a transversal. And 140 degrees and 2y are
corresponding angles. So they're actually going
to equal each other. And you can even eyeball it and
think about, well, that make sense. Those lines are parallel. So 140 degrees is equal to 2y. y is equal to 70. Fair enough. And now let's consider-- let me
switch colors-- that this line right here, this magenta
line and this magenta line, those two are parallel. And we can now view this
as a transversal. Right? If this is a transversal, then
the 70 degrees and this angle are corresponding angles, so
they're going to be equal. So that's 70. And then this angle up here,
that 70 degrees that I just drew, and the x angle, they're
supplementary, right? They add up to 180. So you have x plus 70
is equal to 180. So you get x is equal
to 110 degrees. Oh, no, no, sorry. I made a mistake. I made a mistake. So this angle isn't 70.
y is equal to 70. But this angle is 2y, so
this is 140 degrees. Right? We figured out that this is 140
degrees, so this has to be 140 degrees. And what my brain caught, I was
like, boy, that doesn't look like 70 degrees. 70 degrees would be
a lot narrower. That looks more like 140. But anyway, this is 140. So if this is 140, the
corresponding angle up here is also going to be 140 degrees. And so, I did that wrong. x plus 140, right? x plus
this 140 degrees. The logic is still the same. They're supplementary. Equals 180. So x is equal to 40 degrees. And they wanted to know what? They wanted to know
what y minus x is. So that's 30. 70 minus 40 is 30. And that's choice A. See you in the next video.