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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 22

115-119, pgs. 168-169. Created by Sal Khan.

## Want to join the conversation?

- Wouldn't a better way to deal with 116 is to see, right off the bat, that this is a combinatorics question? i.e. how many of combinations of 2 out of 30 are there, and order doesn't matter? So it's just 30!/2!(28!) = 30*29/2*1 = 15*29 = 435.(3 votes)
- How do you round fractions(2 votes)

## Video transcript

We're on problem 115. Mary's income is 60% more
than Tim's income. So that's 1.6 times
Tim's income. You could write it as Tim's
income plus 60% of Tim's income, but hopefully it's
second nature that this is just 1.6 times Tim's income. So if Mary's income is equal to
1.6 times Tim's income, and Tim's income is 40% percent
less than Juan's income. So Tim's income, if it's 40%
less than Juan's income, that means that it's 60%
of Juan's income. You can do that as Juan's income
minus 40% percent of Juan's income. That's 0.6 times
Juan's income. What percentage of Juan's
income is Mary's income? So let's just do a little
substitution. Substitute this T over here. So you get Mary is equal to
1.6 times 0.6 times Juan. And then what's 6 times 16? 60 plus 36. 96. We have 2 numbers to the
right of the decimal. So it's 0.96 times
Juan's income. So what are they asking? What percentage of Juan's
income is Mary's income? Mary's income is 96%
of Juan's income. And that's choice C. 0.96 is 96% of. Problem 116. Looks like something
I have to draw. Each dot in the mileage table
above represents an entry indicating the distance between
a pair of five cities. Let me try to draw
this out quickly. So we have five cities. It's 1, 2, 3, 4, and 5. And then we have to do
it the other way. 1, 2, 3, and then 4. They have the same cities
on both axes. City A, B, C, D, and E. City A, B, C, D, and E. And they have some dots. That's a dot, a dot, a dot, a
dot, then dots down that way. Each dot in the mileage table
above represents an entry indicating the distance
between a pair of the five cities. If the table were extended to
represent the distances between all pairs of 30 cities,
and each distance were to be represented by only one
entry, how many entries would the table then have? So they're saying, OK, this
is essentially the case with five cities. You don't want to care about the
distance between A and A. They want the distance between
all pairs of 30 cities. So this is the case when you
have n is equal to 5. And the reason why you could say
the distance between A and B is this dot, you could have
also said the distance between B and A is this dot down here. But they didn't draw a dot
there, because they said and each distance were to be
represented by only one entry. So they don't want to double--
if this was 5, this would also be 5. They only want to write
it there once. So relative to 5, how would
we figure this out? So if you think about it, what
you're going to have is-- no matter how big this table
is, the diagonal is going to be empty. You don't have to fill out
the distance between the city and itself. And then if you just continue
the trend that they did, out of what's left over, you're just
going to fill out half of them, because the other half is
just redundant information. So for example, if you have 5
cities, you're going to have 5 times 5 is equal
to 25 squares. How many squares are going
to be in the diagonal? We're going to have exactly
5 squares in the diagonal. 1, 2, 3, 4, 5. So you're going to subtract 5. So you're going to have
20 boxes left that aren't in the diagonal. And then you are going to have
to to fill out half of those. So you're going to fill out
exactly 10 boxes, so 1, 2, 3 4, 5, 6, 7, 8, 9, 10. So let's do the same
exercise with 30. If you have 30 cities, you're
going to have 30 times 30 boxes, which is equal to 900. You're going to take out the
30 boxes that are in the diagonal, so minus 30. So that's equal to 870. And then you are only going to
have to fill out half of those, because you don't want
redundant information. So you are only going to fill
out the top half they way they did it in this diagram. So if you divide this by 2,
you're going to get to what? 870 divided by 2 is 435. And that's choice B. Next question, 117. Which of the following has
a value less than 1? So choice A is 2 times 7/13. 2 times 7 is 14, so that's
greater than 13. So it's not A. B, square root of 10/2. The square root of 10, that's
going to be 3 point something, which is greater than 2. So that's not right. Choice C is 2 over the
square root of 2. The square root of
2 is less than 2. It's 1 point something. So this is going to also have
a value greater than 1. So it can't be this. We're looking for less than 1. Choice D says 1 divided
by 1/2. That's equal to 1 times 2,
which is equal to 2. So I can already tell you the
choice is going to be E. Let's look at it. So they have 9/10 squared. That's 81/100, which
sure enough is definitely less than 1. So the choice is E. Problem 118. The ratio of the length to the
width of a rectangular advertising display is
approximately 3.3:2. So the length is 3.3
and the width is 2. At least that's the ratios. If the width of the display
is 8 meters, what is the approximate length of the
display in meters? So they tell us the length to
the width is approximately 3.3:2, which is approximately
equal to the length to 8 meters. They say the width of the
display is 8 meters. So we just have to solve this
as best as we can for L. The easiest way to think about
it is to go from 2 to 8, you multiply by 4. So to go from 3.3 to L we
have to multiply by 4. So L is equal to 3.3 times 4. What's that? 33 times 4, or 3.3 times 4. 4 times 3 is 12, plus 1 is 13. One number behind the
decimal point. 13.2. So they say approximate,
and the closest one is choice C, 13. Problem 119. The average salary of 15
people in the shipping department of a certain
firm is $20,000. So the average of 15 people
is equal to $20,000. The salary of 5 of the employees
is $25,000 each, so we can even say the average of
those 5 is going to be $25,000 because all of them
make $25,000. And the salary of 4 of the
employees is $16,000 each. The average of 4 of the
employees is $16,000 each. That's the salary, but if you
average 4 people making $16,000, the average is
going to be $16,000. What is the average salary of
the remaining employees? Let's think about this. Let's try to find the average
of all of the employees. So if we find the average of all
15 employees, we're going to sum up all their salaries,
divide by 15. And we know that that is going
to be equal to $20,000. Now we have these 5 guys making
$25,000, so the sum of their salaries is going
to be 5 times $25,000. And then you have these 4 people
making $16,000, so it's plus 4 times 16. I'm just trying to sum up
everyone's salaries up here in the numerator, and I'm going
to divide by 15. And then I'm going
to have what? The average of the
remaining people. How many people are remaining? I've already done 9 people, so
there's 6 people remaining because there's a total
of 15 people. So the average of the 6
remaining people times 6. Whatever their average is, times
6, that'll give you the sum of their salaries. And now we just solve for
the average of those 6. Let's see what we can do. So 5 times 25, that's 125. Plus 4 times 16 is 64. Plus 6 times-- let's
call it A-- 6A is equal to 20 times 15. So what's 20 times 15? 20 times 10 is 200. 20 times 5 is 100. So it's equal to 300. 125 plus 64 is 189 plus
6A, is equal to 300. Subtract 189 from both sides. You get 6A is equal
to 300 minus 189. That's 111. 89, add 11 to it,
you get to 200. And then you have to add
another 100 to get to 300, so it's 111. 300 minus 189. And then how many times
does 6 go into 111? Divide both sides by 6. 6 goes into 111-- 6 goes
into 51 eight times. 8 times 48. 18.5 times. $18,500 is the average
of their salary. And luckily enough, that's
one of the choices. That's choice B. $18,500. We did everything in
[? thousandths ?] so we got 18.5, but
that's $18,500. See you in the next video.