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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 2

7-11, pg. 153. Created by Sal Khan.

## Want to join the conversation?

- Why in #9, after calculating the square root of 0.04, we have to multiply it by 0.2?(2 votes)
- Problem 9 - you arrive at the correct answer and then revert to the incorrect answer in checking your work, no?(2 votes)
- How do I view all the questions before the video plays?(1 vote)
- how many spaces behind the decimal point would there be if the number the number jumped times equal to the base number.(1 vote)
- my teachers are making me do this any help?(1 vote)
- Why can't i look this video?It's always tell me "You've watched 0% of this video".(1 vote)
- i think in problem 11, have little misstake, the question is : Monday, a customer have 8 packages with avarage is 12 3/8 pound. On tuesday, 4 packages is avarage 8, and 4 packages is 15 1/4 pound. i think the answers is:

((8 x 12 3/8) + (4 x8) + (4x 15 1/4)) divide 16 = 12.

>> Average of both days is 12.(1 vote) - From which books are these sums taken from?If there is a reliable book please suggest?(1 vote)
- I think there might be some problem in question 9...Why do we have to multiply it by 0.2?(1 vote)
- what tool (program) are you using to record/demonstrate this solution?(1 vote)

## Video transcript

We're on problem 7. And they drew this graph, so I
guess I'll draw it as well. That's the x-axis. That's the y-axis. Let's see, they draw 1, 2,
and they draw 1, 2, 3. And then the graph goes through
here and then curves up like that and actually
goes through this point right there. It goes through y is equal
to 3, at least the way they drew it. On the graph before, when x
equals 1/2, y is equal to 2. So they're saying that
this point right here is 1/2 comma 2. Fair enough. And when x is equal to
1, y is equal to 1. So they're saying that this
point right here-- I'll do it in a different color, just to
not make it too confusing-- that point right there is
the point 1 comma 1. The graph is symmetrical with
respect to the vertical line at x equals 2. So that means that it's a
mirror image around that vertical line. According to the graph when
x is equal to 3, y should be equal to? So when x is equal to 3, we are
1 away from the line x is equal to 2, or what the line is
a mirror image of, right? So when we were 1 to the
left of it, that's when x is equal to 1. And we learned y
is equal to 1. So for 1 to the right-- and this
is symmetric around this line-- then we're going to have
the same height, right? So when we were 1 to the
left, the height was 1. When we're 1 to the right, the
height should also be 1. So this should be the
point 3 comma 1. So the choice is E. All that other information was
kind of a waste of time. Next problem. Problem 8. When 1/10% of 5,000--
I always like to get these into decimals. 1/10% that equals this divided
by 100, so it's 1/1,000. So it's 0.001. So 0.001 times 5,000 is
subtracted from 1/10 of 5,000. The difference is-- OK. So they're saying 1/10 of 5,000
minus 1/1,000 of 5,000. Right? That's 1/10 of a percent
is 1,000. So you get 500 minus 5, right? That's 495, which is choice D. Problem 9. Which of the following is the
value of the square root of the cube root. So let's see. How many 0's do they have? 1, 2, 3, 4. 1, 2, 3, 4, 6, 4. OK. So there's-- how many
total numbers are behind the decimal point? 1, 2, 3, 4, 5, 6. So it's a cube root. So some number times itself 3
times is equal to 0.0064. And you know whenever you
multiply decimals, you add up the number of spots behind
the decimal point. If I have two numbers that have
two numbers to the right of the decimal point and
I multiply them, I end up with 4. So if you just wanted to eyeball
this, you'd say, well, first of all, what number
to the third power is equal to 64? Well 4 to the third power
is equal to 64, right? It's equal to 16 times 4,
which is equal to 64. So this should have a
4 some place in it. It should look like a 4. It's going to be a decimal
number because obviously the number has gotten smaller
when I took the exponent of this number. And the clue is how
many places are there behind the decimal? 1, 2, 3, 4, 5, 6. And I'm multiplying whatever
this number is times itself 3 times. So in order to get 6 digits
behind the decimal point, that original number probably
has two digits behind the decimal point. So my initial kind of
estimation is going to have a 4 in it. And it's going to be 0.04. And I'm pretty sure
I'm right here. Well, you could try it out for
yourself, but 0.04 times 0.04. You say, oh, 4 times 4 is 16. But I'm going to have now 1,
2, 3, 4 spaces behind the decimal point. And then if you multiply that
times 0.04, 4 times 16 is 64. And now I'm going to have 1,
2, 3, 4, 5, 6 spaces behind the decimal point. And I get 0.000064, which
is exactly there. So the problem now boils down
to square root of 0.04. And you use the exact
same logic. That is equal to 0.2. And try to square 0.2, you'll
get 0.04 right? 0.2 times 0.2. You say 2 times 2 is 4. And I have 1, 2 spaces behind
the decimal point. 0.04, and that's D. Problem 10. Raffle tickets numbered
consecutively from 101 through 350 are placed in a box. What is the probability that a
ticket selected at random will have a number with a hundreds
digit of 2? So we're essentially saying,
out of this collection how many of them have a hundreds
digit of 2. So essentially it's going to be
anything from 200 to 299. Well that's essentially
100 digits, right? Those are 100 numbers
that have a hundreds digit of 2, right? Think about it. 200 by itself would be 1. From 200 to 1 would be 2. So 200 to anything would be
that anything plus 1. So there's 100 numbers
that satisfy it. And then how many numbers
are we picking from? Well, if it was from 100 to
350 it be 250 numbers. But since it's from 101 to 350,
we're picking from a pool of 249 numbers. Right? Although let me think
about that. Let me think about that. Right, we're picking from
a pool of-- I just want to make sure. If it was 101 to 102, we would
be picking from a pool of-- Right, we're picking from a pool
of 249 numbers, so the odds of having a number with a
2 in the hundreds digit will be 100/249. So that is choice E. I'm just making sure that I
haven't made a-- Actually no, I'm going to take that back. If I said from 300 to 350,
that's 51 digits. If I say 101 to 350, that's
not 249 digits. I made a mistake. Because we're including
101, right? So it would actually
be 250 digits. Sorry, it's 250 numbers. We're picking from a
bag of 250 numbers. Think about it. If it was from 101-- you have
to make sure you get your boundaries right on these-- if
we said from 101 to 105, how many are this? This would be 101, 102,
103, 104, and 105. That's 1, 2, 3, 4, 5. It's 5 digits. So it's 105 minus 101 plus 1. So similarly, the bag, the
number of digits we can pick from, is going to be 350
minus 101 plus 1, which is equal to 250. So anyway, out of the bag of
250, 100 numbers satisfy it. So we're going to have 100 in
250 chance of picking the right number. And if we divide the top and the
bottom by 50-- that's the greatest common divisor--
we get 2/5. And that is choice A. And I got a little cautious
because I saw that they had all these choices that were
really close to each other. And they all seemed to test,
like choices D and E, they all seemed to test whether you got
the boundaries right, whether you really counted the
pool correctly. And whether you really
counted the number of digits correctly. And I always have to go through
the exercise of let me make sure whether or not I need
to count 101, whether I can just subtract them,
or whether it's the fractions plus 1. I always have to clarify
that in my own brain. Maybe that was easier for you. Question 11. On Monday, a person mailed 8
packages weighing an average of 12 and 3/8 pounds. And on Tuesday, 4 packages. So this is 8. Weighed that much. And on Tuesday, 4 packages
weighing an average of 15 and 1/4 pounds. What was the average weight in
pounds of all packages the person mailed on both days? So essentially you have to take
the sum of the weights of all of them and divide
by the total, right? So what's the sum of the
weights of all of them? So the sum of the weights of
these 8 packages-- we know their average. So the sum of the weights is
going to be 8 times 12 and 3/8-- that's the sum of all of
these-- plus 4 times the weight of these. Plus 4 times 15 and 1/4. And we're going to divide
that by the total number of packages. Divided by 12. So this is equal
to, let's see. What's 8 times 12? 8 times 12 is 96. And what's 8 times 3/8? So it's 96 plus 3. You could view 12 and
3/8 as 12 plus 3/8. So I kind of just did a
distributive property there. You could turn this
into an improper fraction, if that's easier. And then plus-- what's
4 times 15? It's 60, right? And then 4 times 1/4 is 1. All of that over 12. And so we have 99 plus 61. All of that over 12. Which is equal to 160/12. Let's see. Does 12 go into 160? 12 times 12 is 144. Let me think about this. So let me just divide
top and bottom by 4. So you get 40/3. And let's see. All of the answers they have
it in terms of fractions. So 3 goes into 40
how many times? It goes into 13, right? 13 times 3 is 39. So it's 13 with remainder 1. So 13 and 1/3 times,
so A is the answer. So the average of all of the
packages on both days are 13 and 1/3 pounds. See you in the next video.