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Intern  Joined: 02 Jan 2011
Posts: 8

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I am not so certain whether I am right but I would go with D in #9.

IF x<0 and y<0 then sqrt(x^2)/x=|x|/x, then -x/x=-1; because there is a minus in front of x, than -(-x) should be positive.

sqrt two negative y gives square root of y^2, than because y<1, y is negative as well. -y.

From the expression we've got -1+y, thus D should be a correct answer.
Senior Manager  Joined: 23 Oct 2010
Posts: 318
Location: Azerbaijan
Concentration: Finance
Schools: HEC '15 (A)
GMAT 1: 690 Q47 V38 ### Show Tags

Bunuel, could u please tell us which question is still unanswered or answered wrong by all of us? I want to think on such questions more, before u post solutions.

and btw, do u read our solutions or just only answers? I wonder whether my way of thinking was ok. in some cases I tried to use another method not to repeat others. I just wonder whether it worked, or it was just coincidence
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Happy are those who dream dreams and are ready to pay the price to make them come true

I am still on all gmat forums. msg me if you want to ask me smth
Math Expert V
Joined: 02 Sep 2009
Posts: 59561

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LalaB wrote:
Bunuel, could u please tell us which question is still unanswered or answered wrong by all of us? I want to think on such questions more, before u post solutions.

and btw, do u read our solutions or just only answers? I wonder whether my way of thinking was ok. in some cases I tried to use another method not to repeat others. I just wonder whether it worked, or it was just coincidence

I do read all the solutions and award +1 Kudos if there is at least one correct explanation in the post.

As for your other question: I don't see a solution for #7, so you can try this one.
Intern  Joined: 02 Jan 2011
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Was it the wrong solution of #9. It seems to me that my post was removed Math Expert V
Joined: 02 Sep 2009
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SergeNew wrote:
Was it the wrong solution of #9. It seems to me that my post was removed Even if it were wrong I wouldn't remove it, it's just on the second page: baker-s-dozen-128782-20.html#p1056538
Senior Manager  Joined: 23 Oct 2010
Posts: 318
Location: Azerbaijan
Concentration: Finance
Schools: HEC '15 (A)
GMAT 1: 690 Q47 V38 ### Show Tags

2
1
Bunuel wrote:

As for your other question: I don't see a solution for #7, so you can try this one.

ok my answer to the q7 is D , because D is the leader of the day hehe kidding
the answer is really D ,because-

2/3(L+A+W) =F
3/7(F+W+A)=L
4/11(F+W+L)=A
total=360 (I deleted the 0s just to make a number easy)
W=?

2/3(L+A+W) =F means F=2/3(360-F) =>(2/3)*360=5/3F=> F=144
3/7(F+W+A)=L means L=3/7(360-L) => L=108
4/11(F+W+L)=A means A=4/11(360-A)=>A=96

W=360- (L+A+F)=360-144-108-96=12

or W=120 000 (return the 0s which were deleted before)
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Happy are those who dream dreams and are ready to pay the price to make them come true

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Veritas Prep GMAT Instructor V
Joined: 16 Oct 2010
Posts: 9848
Location: Pune, India

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utkarshlavania wrote:
[quote2]9. If x and y are negative numbers, what is the value of $$\frac{\sqrt{x^2}}{x}-\sqrt{-y*|y|}$$?
A. 1+y
B. 1-y
C. -1-y
D. y-1
E. x-y

Answer to 9th question If x and y are negative numbers, what is the value of $$\frac{\sqrt{x^2}}{x}-\sqrt{-y*|y|}$$?[/b]
Sqrtx^2/x-sqrt(-y*|y|) = |x|/x-|y|= -1-y

Responding to a pm:

First of all, I would suggest you to put x = -2 and y = -3 and see what you get.

$$\frac{\sqrt{x^2}}{x}-\sqrt{-y*|y|}$$

$$\frac{\sqrt{(-2)^2}}{-2}-\sqrt{-(-3)*|-3|}$$

$$\frac{2}{-2}-\sqrt{9}$$ (because square root will always be positive)

-1 - 3 = -1 + y (since y = -3)

I see you have everything correct till the last step: |x|/x-|y|

|x|/x will be -1 which is fine.
Do you remember how we define mods?
We say:
|x| = x if x >= 0
|x| = -x if x < 0

Why? If x >= 0 e.g. say x = 5,
|x| = x = 5
Instead, if x < 0 e.g. say x = -6,
is |x| = x? No! mods are never negative. So |x| = -x = -(-6) = 6 (Since x itself is negative, mod of x is negative of x which becomes positive.)

Therefore, if we know that y is negative, what is the value of |y|?
|y| = -y

Therefore, |x|/x-|y| = -1 -(-y) = -1 + y

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Karishma
Veritas Prep GMAT Instructor

Intern  Joined: 03 Sep 2010
Posts: 15

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@karishma

then why is |x|/x =-1 , should not it be 1 as |x|=-x and denominator is negative as well
Math Expert V
Joined: 02 Sep 2009
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utkarshlavania wrote:
@karishma

then why is |x|/x =-1 , should not it be 1 as |x|=-x and denominator is negative as well

If $$x<0$$ then $$|x|=-x$$ and $$\frac{|x|}{x}=\frac{-x}{x}=-1$$, ($$-\frac{x}{x}=-1$$ no matter whether $$x$$ is negative or positive).
Veritas Prep GMAT Instructor V
Joined: 16 Oct 2010
Posts: 9848
Location: Pune, India

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utkarshlavania wrote:
@karishma

then why is |x|/x =-1 , should not it be 1 as |x|=-x and denominator is negative as well

You are right that |x|=-x since x is negative but tell me, what is -x, negative or positive? Negative of negative gives you positive, right? So -x must be positive. Now, if x is negative,
-x/x must be positive/negative giving you -1.

You are confusing yourself too much with negatives and positives. Just think of it this way:

|x|/x = -x/x (By definition, since |x| = -x when x < 0)
-x/x = -1 ( x and x get canceled here leaving you with -1)
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Karishma
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Math Expert V
Joined: 02 Sep 2009
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SOLUTIONS:

1. A password on Mr. Wallace's briefcase consists of 5 digits. What is the probability that the password contains exactly three digit 6?

A. 860/90,000
B. 810/100,000
C. 858/100,000
D. 860/100,000
E. 1530/100,000

Total # of 5 digit codes is 10^5, notice that it's not 9*10^4, since in a code we can have zero as the first digit.

# of passwords with three digit 6 is $$9*9*C^3_5=810$$: each out of two other digits (not 6) has 9 choices, thus we have 9*9 and $$C^3_5$$ is ways to choose which 3 digits will be 6's out of 5 digits we have.

$$P=\frac{favorable}{total}=\frac{810}{10^5}$$

Math Expert V
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2. If $$y=\frac{(3^5-3^2)^2}{(5^7-5^4)^{-2}}$$, then y is NOT divisible by which of the following?
A. 6^4
B. 62^2
C. 65^2
D. 15^4
E. 52^4

$$y=\frac{(3^5-3^2)^2}{(5^7-5^4)^{-2}}=(3^5-3^2)^2*(5^7-5^4)^2=3^4*(3^3-1)^2*5^8*(5^3-1)^2=3^4*26^2*5^8*124^2=2^6*3^4*5^8*13^2*31^2$$.

Now, if you analyze each option you'll see that only $$52^4=2^8*13^4$$ is not a factor of $$y$$, since the power of 13 in it is higher than the power of 13 in $$y$$.

Math Expert V
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3. For the past k days the average (arithmetic mean) cupcakes per day that Liv baked was 55. Today Bibi joined and together with Liv they baked 100 cupcakes, which raises the average to 60 cupcakes per day. What is the value of k?
A. 6
B. 8
C. 9
D. 10
E. 12

Total cupcakes for k days was 55k, which means that total cupcakes for k+1 days was 55k+100. The new average is (55k+100)/(k+1)=60 --> 55k+100=60k+60 --> k=8

Math Expert V
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8
4. What is the smallest positive integer $$k$$ such that $$126*\sqrt{k}$$ is the square of a positive integer?
A. 14
B. 36
C. 144
D. 196
E. 441

$$126=2*3^2*7$$, so in order $$126*\sqrt{k}$$ to be a square of an integer $$\sqrt{k}$$ must complete the powers of 2 and 7 to even number, so the least value of $$\sqrt{k}$$ must equal to 2*7=14, which makes the leas value of $$k$$ equal to 14^2=196.

Math Expert V
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48
5. There are 7 red and 5 blue marbles in a jar. In how many ways 8 marbles can be selected from the jar so that at least one red marble and at least one blue marble to remain in the jar?
A. 460
B. 490
C. 493
D. 455
E. 445

Total ways to select 8 marbles out of 7+5=12 is $$C^8_{12}$$;
Ways to select 8 marbles so that zero red marbles is left in the jar is $$C^7_7*C^1_5$$;
Ways to select 8 marbles so that zero blue marbles is left in the jar is $$C^5_5*C^3_7$$;

Hence ways to select 8 marbles so that at least one red marble and at least one blue marble to remain the jar is $$C^8_{12}-(C^7_7*C^1_5+C^5_5*C^3_7)=495-(5+35)=455$$.

Math Expert V
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38
6. A pool has two water pumps A and B and one drain C. Pump A alone can fill the whole pool in x hours, and pump B alone can fill the whole pool in y hours. The drain can empty the whole pool in z hours, where z>x. With pumps A and B both running and the drain C unstopped till the pool is filled, which of the following represents the amount of water in terms of the fraction of the pool which pump A pumped into the pool?
A. $$\frac{yz}{x+y+z}$$

B. $$\frac{yz}{yz+xz-xy}$$

C. $$\frac{yz}{yz+xz+xy}$$

D. $$\frac{xyz}{yz+xz-xy}$$

E. $$\frac{yz+xz-xy}{yz}$$

With pumps A and B both running and the drain unstopped the pool will be filled in a rate $$\frac{1}{x}+\frac{1}{y}-\frac{1}{z}=\frac{yz+xz-xy}{xyz}$$ pool/hour. So, the pool will be filled in $$\frac{xyz}{yz+xz-xy}$$ hours (time is reciprocal of rate).

In $$\frac{xyz}{yz+xz-xy}$$ hours A will pump $$\frac{1}{x}*\frac{xyz}{yz+xz-xy}=\frac{yz}{yz+xz-xy}$$ amount of the water into the pool.

Math Expert V
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8. A set A consists of 7 consecutive odd integers. If the sum of 5 largest integers of set A is -185 what is the sum of the 5 smallest integers of set A?
A. -165
B. -175
C. -195
D. -205
E. -215

Say 7 consecutive odd integers are: $$x$$, $$x+2$$, $$x+4$$, $$x+6$$, $$x+8$$, $$x+10$$, $$x+12$$.

Question: $$x+(x+2)+(x+4)+(x+6)+(x+8)=5x+20=?$$

Given: $$(x+4)+(x+6)+(x+8)+(x+10)+(x+12)=-185$$ --> $$(x+4)+(x+6)+(x+8)+(x+10)+(x+12)=5x+40=-185$$ --> $$(5x+20)+20=-185$$ --> $$5x+20=-205$$

Math Expert V
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9. If x and y are negative numbers, what is the value of $$\frac{\sqrt{x^2}}{x}-\sqrt{-y*|y|}$$?
A. 1+y
B. 1-y
C. -1-y
D. y-1
E. x-y

Note that $$\sqrt{a^2}=|a|$$. Next, since $$x<0$$ and $$y<0$$ then $$|x|=-x$$ and $$|y|=-y$$.

So, $$\frac{\sqrt{x^2}}{x}-\sqrt{-y*|y|}=\frac{|x|}{x}-\sqrt{(-y)*(-y)}=\frac{-x}{x}-\sqrt{y^2}=-1-|y|=-1+y$$

Math Expert V
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10. If x^2<81 and y^2<25, what is the largest prime number that can be equal to x-2y?
A. 7
B. 11
C. 13
D. 17
E. 19

Notice that we are not told that $$x$$ and $$y$$ are integers.

$$x^2<81$$ means that $$-9<x<9$$ and $$y^2<25$$ means that $$-5<y<5$$. Now, since the largest value of $$x$$ is almost 9 and the largest value of $$-2y$$ is almost 10 (for example if $$y=-4.9$$), then the largest value of $$x-2y$$ is almost 9+10=19, so the actual value is less than 19, which means that the largest prime that can be equal to $$x-2y$$ is 17. For example: $$x=8$$ and $$y=-4.5$$.

Math Expert V
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3
11. In an infinite sequence 1, 3, 9, 27, ... each term after the first is three times the previous term. What is the difference between the sum of 13th and 15th terms and the sum of 12th and 14th terms of the sequence?
A. 10*3^11
B. 20*3^11
C. 10*3^12
D. 40*3^11
E. 20*3^12

You don't need to know geometric progression formula to solve this question. All you need is to find the pattern:

$$b_1=1=3^0$$;
$$b_2=3=3^1$$;
$$b_3=9=3^2$$;
$$b_4=27=3^3$$;
...
$$b_n=3^{n-1}$$;

$$b_{13}+b_{15}-(b_{12}+b_{14})=3^{12}+3^{14}-3^{11}-3^{13}=3^{11}(3+3^3-1-3^2)=20*3^{11}$$ Re: Baker's Dozen   [#permalink] 13 Mar 2012, 04:00

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# Baker's Dozen  