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### Course: GMAT > Unit 1

Lesson 1: Problem solving- GMAT: Math 1
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# GMAT: Math 26

133-137, pg. 170. Created by Sal Khan.

## Want to join the conversation?

- For question 135, I solved by multiplying the path choices; 2*2*3=12. Is that was Sal was basically saying to do? I was a little confused by his explanation. Thanks!(6 votes)
- Yea the OG 12th edition uses this method as well.(4 votes)

- Question 137. In problem set up, you state 10 to the 4th minus 10 to the 12th, yet you put 10 to the 2nd in the written example. Which is correct?Also, rather than going through the lengthy process of using the distributive property rule, why not use order of operations and perform the work in the first parenthesis and multiply by the 2nd factor?(2 votes)
- In a family there were x people. The expenditure of rice per month in this family is directly proportional to 5 time the square of number of people in the family. If the elder son left the famiy to stuy in Canada, there was decrease in consumption of 95 kg rice per month. Then the value of x is?(1 vote)
- Question 137. In problem set up, you state 10 to the 4th minus 10 to the 12th, yet you put 10 to the 2nd in the written example. Which is correct? Also, rather than going through the lengthy process of using the distributive property rule, why not use order of operations and perform the work in the first parenthesis and multiply by the 2nd factor?(0 votes)
- Jim, I tried the work in the parenthesis first, and I got 11.88 which is close to 12, but interesting that it wasn't a straight-forward 12 like Sal got. Just remember that you can't subtract 10^4 - 10^2 straight-up, you've got to factor out 10^2; 10^2(100 - 1) (0.0012)(0 votes)

- Hi all, problem 137: I am confused that we did not follow the rule of order of operations. Why didn't we solve the inside of the parenthesis first and do the multiplication? Thanks for your answers!(0 votes)
- You could, but we see a cleaner, easier method in this case. If you reverse the values, you can rewrite the problem as such:

x(10^4 -10^2) where x = .00121212.

We're just simplifying: 10^4x - 10^2x

If you did it the other way, you'd have .001212*9900 --- it'd be much more time consuming to arrive at the correct answer.(0 votes)

## Video transcript

Problem 133. A certain country had a total
annual expenditure of 1.2 times 10 to the 1/12
power last year. If the population of the country
was 240 million last year, what was the per
capita expenditure? 240 million, what is that? A million is 10 to
the sixth power. So this is 240 times
10 to the sixth. And that's the same thing as
24 times 10 to the seventh. I just took 10 from here
and multiply 10 there. And that is equal to 2.4 times
10 to the eighth power. Divide this by 10 and
multiply this by 10. You get the same number. So 2.4 times 10 to the eighth. So the per capita income is
going to be equal to the total annual expenditure, so 1.2 times
10 to the 12th, divided by the population, 2.4 times
10 to the eighth. 1.2 divided by 2.4, that's equal
to 0.5, that's exactly half of that, times 10
to the 12th divided by 10 to the eighth. You just subtract the
exponents, so that's 10 to the fourth. And that's not one of the
answers, so we can multiply this 10 and divide this by 10. So this becomes 5 times
10 to the third. Ten to the third is 1,000, so
5 times 1,000 is equal to $5,000 in annual expenditure
per capita. And that's choice E. Problem 134. A certain rectangular
window is twice as long as it is wide. So if this is the width, its
length is 2 times the width. If its perimeter is 10 feet, and
its dimensions are, so its perimeter going to be w plus
w plus the 2 lengths. So its perimeter is going to be
2w plus 2w plus w plus w. And that's going to
be equal to 10. If we add up all the w's,
that's 2, 4, 5, 6. So we get 6w is equal to 10. w is equal to 10/6, which
is equal to 5/3 feet. And so its dimensions are
5/3 and then the 2w. So the length is 2 times
that, which is 10/3. So it's 10/3 by 5/3. And that's choice B. They say 5/3 by 10/3,
but same thing. So choice B. Problem 135. This is interesting. Let's see if I can draw
this diagram. So to start, this is x, then
it goes there and has a diamond like that, and
it goes like that. And we have another diamond. I don't know what this is all
for yet, but it looks interesting. It goes like that, then
goes like that. There's a diamond there, looks
like there, looks like there. There's a line that goes like
that, goes like that, like that, and like that. And then finally, we're at y. The diagram above shows of
various paths along which a mouse can travel from point x
where does it lead to point y, where there's a reward
with a food pellet. How many different paths from
x to y can the mouse take if it goes directly from x to y
without retracing any point along the path? There's only one way. It has to go here. So we're only at 1
path right now. And it can either go this
way to get here, or it could go this way. So to get here, there's how
many different paths? Like that, or like that. So at this point there's 2
ways to get right there. There's 1 way to get here,
2 ways to get here. Now, if there's 2 ways to get
here, that means that there's 2 ways to get here. Now how many ways are
there to get here? There's each of the 2 ways to
get there, and now we each of those in combination with each
of these 2 ways, so there's 4 ways to get here. You could think of
it like this. You could go to the top 1
and the top, that's 1. Top and the bottom, 2. Bottom and the bottom, 3. Bottom and the top, 4. So you could just multiply how
many ways it takes to get here, and then how many more
ways to get from here to here. From here to here, there's 2
possible ways, and there's already 2 ways to get here, so
you could multiply the 2. So there's 4 ways to get here,
4 ways to get here. To get from here to here, there
are 3 ways, 1, 2, and 3. But since there are already 4
ways to get here, with each of those 4 ways you can go
in 3 different ways. So we multiply by 4 times 3,
so there's 12 ways to get right here. If there's 12 ways to
get here, there's 12 ways to get to y. So the answer is C, 12. Problem 136. If the operation circle with
a dot in it is defined by x circle with a dot y, is equal to
the square root of xy, for all positive numbers x and y. So they're saying x and
y are greater than 0. That's good, because that
ensures we have a positive under the square root sign. Then they want to know what 5
circle with a dot 45 circle with a dot 60 is equal to. 5 circle 45 five, that's
equal to the square root of 5 times 45. That's that, and whatever that
is we're circling with that. So what you do when you have x
circle y, you multiply the 2 and then you take
the square root. So we would multiply 60 times
this and take the square root of the whole thing. Let's just simplify this. This right here is equal to the
square root of 5 times 45, which doesn't look simplifiable
at face value. But if you think about,
45 is 9 times 5. You just want to take
out any perfect squares you have there. So that's equal to 5
times 9 times 5. And that's equal to square
root of 25 times 9. And that is equal to the square
root of 25 times the square root of 9, and that's
equal to 5 times 3, which is equal to 15. So this simplifies to 15. And 15 dot circle, whatever you
want to call it, 60, is equal to the square root
of 15 times 60. And let's see if we can somehow
turn this into some type of a perfect square. Let's rewrite this as-- 60 is
the same thing as 15 times 4. So this is equal to the square
root of 15 times 15 times the square root of 4. The square root of 15 times 15
is going to be 15 times 2, which is equal to 30. And that's choice A. Problem 137. I'll do it in yellow. A bar over a sequence of digits
in a decimal indicates that the sequence repeats
indefinitely. What is the value of 10 to the
fourth minus 10 to the 12 times 0.0012, and then this 12
will just keep repeating over and over again. So let's see what we can do. What's 10 to the fourth? Let's think about it this way. Let's distribute this. So what's 10 to the fourth? That's 10,000. So what's 10,000 times
this number, 0.0012? This 12 keeps repeating,
so we'll think about that in a second. So what's 12 times 10,000? It's 120,000. It's 12 with four
0's, 1, 2, 3, 4. And we have 4 numbers behind
the decimal point, so it gets us there. You might say OK, we're done. It's 12. But it's actually
not 12, because it's actually 12121212. The 12's just keep going. It would actually be 121212, and
then 12's keep repeating on like that. I think you understand
why, right? This would be if we just had 1
12 times 10,000, but if the 12's kept going, then these
wouldn't be 0's, these would be 12's. Hopefully that make sense to
you, maybe there's an easy way to think about it. So what's 100? We'll use the same logic. What's 100 times 0.0012? Actually, I probably could have
done it a simpler way, but let's just continue
this way. So 12 times 100 1,200, and we
have 4 numbers behind the decimal point, 1, 2, 3, 4. And of course, it's not just
0.12, it'll just keep going. 0.1212 will just keep going. So we have this times
this, which is this minus this times this. I'm just distributing it. So if I just subtract this
from that, minus 0.12-- I wrote this right. This was 12, because we said
12 times 10,000 is 120,000. We had 1, 2, 3, 4 behind
the decimal. So it's 12. But since we had multiple 12's
behind it, it becomes 12.12. I don't know what I was doing
with this right here. So let me write that again. Sorry I'm doing this
a little bit messy. So 0.0012 repeating times 10 to
the fourth is going to be equal to 12.12 repeating. And then we're going to subtract
0.0012 repeating times 10 squared, and that is
equal to 0.12 repeating. So if I subtract out the 0.12
repeating from 12.12 repeating, I'm just going
to be left with 12. And hopefully that's
one of the answers. It is. Choice E. See you in the next video.