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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
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- 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 7

36-41, pgs. 156-157. Created by Sal Khan.

## Want to join the conversation?

- #41 answer = 2/5 am i not correct?(39 votes)
- For #37, why is the answer II only? I'm struggling to understand how that is answer he arrived at.(8 votes)
- As Sal demonstrated, if a number is divisible by both 5 and 7, then it will be divisible by 35 * some factor (he used k in the video). So to meet the requirement, 35 is the only answer the question. For example, 105 is divisible by both 5 and 7, as well as, 35. However, 105 is not divisible by 70 so that answer does not work. I would be interested to see what the answer key in the book actually says.(6 votes)

- Can someone explain why on question 37 III. 70 is not correct?(4 votes)
- We are told that the mystery integer is positive and is divisible by both 5 and 7. So, it
`could`

be

35, 70, 105, 140, 175, 210, and so on.

But of this list, only**every other**number is divisible by 70. So 70 CANNOT be correct.

In order for the mystery integer to ALSO be divisible by 70, we would have to be told that the mystery integer is ALSO divisible by 2, in addition to being divisible by 5 and 7--and they didn't say that. That chops out 70 as a possible answer.(3 votes)

- is there a text book for this?(1 vote)
- Yes. Go to the very first gmat math video. Someone posted a link to download the pdf. You can also buy the book.(3 votes)

- Q41: 2=5t

t=2/5 ( how can it be 5/2?)(2 votes) - At around the5:50mark in question 38, why did Sal do b - 4 = 3? The 3 came from 3x but how did Sal know to do that?(3 votes)
- As Sal demonstrated, if a number is divisible by both 5 and 7, then it will be divisible by 35 * some factor (he used k in the video). So to meet the requirement, 35 is the only answer the question. For example, 105 is divisible by both 5 and 7, as well as, 35. However, 105 is not divisible by 70 so that answer does not work. I would be interested to see what the answer key in the book actually says.(2 votes)
- why is it 3.2 if u thing as 4.1 over 8.4(2 votes)
- Thank you, I have seen one error of answer 41 that is 2=5t, so that t=2/5. But the answer show here is t=5/2. please rectify it, thank you very much.(2 votes)
- do you still answer the question we ask in the comments(2 votes)

## Video transcript

We're on problem 36. They've drawn a little
graph here. So let's draw it up
for ourselves. So that's the y-axis. That is the x-axis. And then they have this point. Let's see, minus 1. They say, 1, 2, 3. Let's see. 1, 2. And then they have this line
segment PQ that goes from 0, minus 1. Let me draw that in
a different color. It goes from 0, minus
1 to 3 comma 2. So it goes from there, so
3, so roughly there. I think that's about as
good as I can do. And the way they drew it,
it intersects 1 here. And so this would be 3. This is the point 3 comma 2. And they say in the figure
above, the point on segment PQ, so they say this is point
P, this is point Q. The point on PQ that is twice
as far from P as from Q. So the point is on this segment,
so this point is someplace around here. We could find points that are
away from the segment that are twice as far from P as
from Q, but they say it's on this segment. So let's think about
it a little bit. If we just look at how do we
get from P to Q, we go 3 to the right, right? x increases
by 3 and y increases by 3, right? So if we wanted to go 2/3 along
the way, what if we just increased x by 2 to get to x is
equal to 2 and increased y by 2, right? So the x's and y's only
increase by 2/3 of the way to Q. So then that would
be the point. x is 2 and then y is 1. That would be the point
2 comma 1, which is B. Next question. 37. If a positive integer n is
divisible by both 5 and 7, than n must be divisible by
which of the following? Let's see, they give
us a couple of choices: I, II and III. So they say it has to
be divisible by 12. No, that's not the case. 35 is divisible by 5 and 7. It's not divisible by 12. 35. Well, sure. If you're divisible by 5 and
7, that means it's equal to some constant times
5 times 7, right? And this could be a bunch of
numbers multiplied together. But that means it's equal to 35
times some other integer, and that integer could have
a bunch of other factors. So, right. It has to be divisible by 35. And statement III is 70. Well, no, 35 is divisible
by 5 and 7, but it's not divisible by 70. So statement III is not right. So it's II only. So that's C. Question 38. If 4 is one solution to the
equation x squared plus 3x plus k is equal to 10,
so 4 is one solution. So that means if I were to
factor this, it'll be x minus 4 times x plus or minus
something else, because we know that x minus 4 equals 0,
which tells us that x is equal to 4 is one of the solutions. So what are they going
to ask us? They're going to say, where k
is a constant, what is the other solution? So let's think about
it a little bit. Well, actually, let me rewrite
it a little different. I didn't realize that they
said this is equal to 10. So let's subtract 10
from both sides. We get x squared plus 3x-- you
essentially want to find the roots of this quadratic-- plus
k minus 10 is equal to 0. And you want to factor
it, right? You could factor
any quadratic. It's going to be x plus
a times x plus b. And they're telling us
that 4 is one of the solutions, right? 4 is one of the solutions,
so minus 4 could be a. So this would be x minus 4 times
some other number, x plus b, is equal to 0. And I said positive
4 in the solution. Why did I put a minus 4 here? Because think about it. If we factor this quadratic and
we get this, that's tells us either x minus 4 is
0 or x plus b is 0. If x minus 4 is 0, then the
solution is x is equal to 4, so that's why I wrote
x minus 4 there. But anyway, let's figure
out what b could be. So if we multiply this out to
try to pattern match it with this, we get x times x is x
squared minus 4x plus b times x, and then you have minus
4 times b, so minus 4b. So then this is equal to x
squared, and then we could write it plus b minus
4 times x. I just factored an x out of
these two terms, right? And so you get b minus 4
minus 4b is equal to 0. So now you just pattern
match it. You say b minus 4-- x squared
equals x squared. b minus 4x has to be
equal to 3x, right? We're just pattern matching. That's the only first-degree
x term there. So b minus 4 has to be equal--
x has to be equal to 3x. We don't even care about k. So b minus 4 has to be equal
to 3, or b is equal to 7. Oh, right, right. So they want to know what
the other solution is. They don't want to
know what b is. And I was looking and
was like, oh, wow, there isn't a 7 there. There's a negative 7. So that tells us that the
factorization of this problem-- I would have gotten
that wrong if they put a 7 there, because I wasn't
careful-- is x minus 4 times x plus 7 is equal to 0, right? So we already said we could say
x minus 4 is equal to 0. And that's where we get
the 4 solution. Or we could say x plus
7 is equal to 0. Subtract 7 from both sides. The other solution is x
is equal to minus 7. That was a careless mistake I
almost made, but I got saved by the choices. So that's choice A. Next problem. 39. If x is equal to negative
3, what is the value of minus 3x squared? So I don't know, maybe they're
trying to test your ability to do order of operations
or something. So this is equal to minus 3
times minus three squared, right? x squared. So this is equal to minus
3 times positive 9. Minus 3 squared is positive 9,
which equals minus 27, which is choice A. Next question. 40. 29 squared plus 29, over
29 is equal to? Well, let's just divide
the numerator and the denominator by 29. What's 29 squared? This is the same thing as 29
squared over 29 plus-- I'm just going to separate these
two terms out-- plus 29/29. 29 squared divided by 29. x squared divided by x is x. 29 squared divided
by 29 is 29. 29 divided by 29 is 1,
so it equals 30. So that is choice E. Problem 41. If x is equal to 1 minus 3t and
y is equal to 2t minus 1. So I'm going to write
it like this. minus 1 plus 2t. I just want to put the t's
under each other, right? 2t minus 1. For what value of t
does x equal y? OK, so let's just set x equal
y and solve for t. So we could just say if x is
equal to y, we get 1 minus 3t has to equal this, which
is 2t minus 1. Let's add 3t to both sides. So you get 1-- 3t goes
away from there-- equals 5t, minus 1. Add 1 to both sides. You get 2 is equal to 5t. Divide both sides by 5. You get t is equal to 5/2,
which is choice A. And I'm almost out of time. The next one has a diagram,
which always takes me some time to draw, so I'll see
you in the next video.