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12 Easy Pieces (or not?)

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Re: 12 Easy Pieces (or not?)  [#permalink]

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New post 27 Mar 2017, 00:32
Bunuel wrote:
9. Julie is putting M marbles in a row in a repeating pattern: blue, white, red, green, black, yellow, pink. If the row begins with blue marble and ends with red marble, then which of the following could be the value of M?
A. 22
B. 30
C. 38
D. 46
E. 54

There are total of 7 different color marbles in a pattern. Now, as the row begins with blue marble and ends with red marble (so ends with 3rd marble in a pattern) then M=7k+3. The only answer choice which is multiple of 7 plus 3 is 38=35+3.

Answer: C.


Hi Bunuel, I tried to refer other posts for this answer but I did not understand. Can you please elaborate this? how M=7k+3..
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New post 27 Mar 2017, 00:48
RMD007 wrote:
Bunuel wrote:
9. Julie is putting M marbles in a row in a repeating pattern: blue, white, red, green, black, yellow, pink. If the row begins with blue marble and ends with red marble, then which of the following could be the value of M?
A. 22
B. 30
C. 38
D. 46
E. 54

There are total of 7 different color marbles in a pattern. Now, as the row begins with blue marble and ends with red marble (so ends with 3rd marble in a pattern) then M=7k+3. The only answer choice which is multiple of 7 plus 3 is 38=35+3.

Answer: C.


Hi Bunuel, I tried to refer other posts for this answer but I did not understand. Can you please elaborate this? how M=7k+3..


The row begins with blue marble and ends with red marble. What cases can we have?

{blue, white, red} = 7*0 + 3
{blue, white, red, green, black, yellow, pink} {blue, white, red} = 7*1 + 3
{blue, white, red, green, black, yellow, pink} {blue, white, red, green, black, yellow, pink} {blue, white, red} = 7*2 + 3
{blue, white, red, green, black, yellow, pink} {blue, white, red, green, black, yellow, pink} {blue, white, red, green, black, yellow, pink} {blue, white, red} = 7*3 + 3
...

As you can see in any case the number of marbles must be a multiple of 7 plus 3.

Hope it's clear.
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Re: 12 Easy Pieces (or not?)  [#permalink]

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New post 29 Mar 2017, 13:59
Bunuel wrote:
12. If \({-\frac{1}{3}}\leq{x}\leq{-\frac{1}{5}}\) and \({-\frac{1}{2}}\leq{y}\leq{-\frac{1}{4}}\), what is the least value of \(x^2*y\) possible?
A. -1/100
B. -1/50
C. -1/36
D. -1/18
E. -1/6

To get the least value of \(x^2*y\), which obviously will be negative, try to maximize absolute value of \(x^2*y\), as more is the absolute value of a negative number "more" negative it is (the smallest it is).

To maximize \(|x^2*y|\) pick largest absolute values possible for \(x\) and \(y\): \((-\frac{1}{3})^2*(-\frac{1}{2})=-\frac{1}{18}\). Notice that: -1/18<-1/36<-1/50<-1/100, so -1/100 is the largest number and -1/18 is the smallest number (we cannot obtain -1/6 from x^2*y or else it would be the correct answer).

Answer: D.


Is it not the other way around where -1/100 is the least number and -1/18 is the max negative number. I would have picked A as the answer.
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Re: 12 Easy Pieces (or not?)  [#permalink]

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New post 29 Mar 2017, 21:38
vs224 wrote:
Bunuel wrote:
12. If \({-\frac{1}{3}}\leq{x}\leq{-\frac{1}{5}}\) and \({-\frac{1}{2}}\leq{y}\leq{-\frac{1}{4}}\), what is the least value of \(x^2*y\) possible?
A. -1/100
B. -1/50
C. -1/36
D. -1/18
E. -1/6

To get the least value of \(x^2*y\), which obviously will be negative, try to maximize absolute value of \(x^2*y\), as more is the absolute value of a negative number "more" negative it is (the smallest it is).

To maximize \(|x^2*y|\) pick largest absolute values possible for \(x\) and \(y\): \((-\frac{1}{3})^2*(-\frac{1}{2})=-\frac{1}{18}\). Notice that: -1/18<-1/36<-1/50<-1/100, so -1/100 is the largest number and -1/18 is the smallest number (we cannot obtain -1/6 from x^2*y or else it would be the correct answer).

Answer: D.


Is it not the other way around where -1/100 is the least number and -1/18 is the max negative number. I would have picked A as the answer.


No.

-1/18 = ~-0.06 and -1/100 = -0.01

-0.06 < -0.01

Image

As you can see -1/100 is to the right of -1/18.

Attachment:
MSP3701ihg50f15748di7300006532e8b04fda4c6c.gif
MSP3701ihg50f15748di7300006532e8b04fda4c6c.gif [ 1.41 KiB | Viewed 1299 times ]

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Re: 12 Easy Pieces (or not?)  [#permalink]

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New post 28 May 2017, 22:08
Bunuel wrote:
11. The numbers {1, 3, 6, 7, 7, 7} are used to form three 2-digit numbers. If the sum of these three numbers is a prime number p, what is the largest possible value of p?
A. 97
B. 151
C. 209
D. 211
E. 219

What is the largest possible sum of these three numbers that we can form? Maximize the first digit: 76+73+71=220=even, so not a prime. Let's try next largest sum, switch digits in 76 and we'll get: 67+73+71=211=prime.

Answer: D.


Was it implicit that we could only use available digits only once ? I wasted time using same digits multiple times to form different no . Though I ended up with ( 77,73,61 ) which too gave 211 but I arrived at it by brute force. Was there something in the language that I should be careful to identify such scenarios so I don't make this mistake again ?
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New post 28 May 2017, 23:34
booksknight wrote:
Bunuel wrote:
11. The numbers {1, 3, 6, 7, 7, 7} are used to form three 2-digit numbers. If the sum of these three numbers is a prime number p, what is the largest possible value of p?
A. 97
B. 151
C. 209
D. 211
E. 219

What is the largest possible sum of these three numbers that we can form? Maximize the first digit: 76+73+71=220=even, so not a prime. Let's try next largest sum, switch digits in 76 and we'll get: 67+73+71=211=prime.

Answer: D.


Was it implicit that we could only use available digits only once ? I wasted time using same digits multiple times to form different no . Though I ended up with ( 77,73,61 ) which too gave 211 but I arrived at it by brute force. Was there something in the language that I should be careful to identify such scenarios so I don't make this mistake again ?


We are given a data set which has three 7's in it. If we could use each number in the set multiple times, then why 7's were written three times?
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Re: 12 Easy Pieces (or not?)  [#permalink]

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New post 11 Jul 2017, 04:44
Bunuel wrote:
2. If x is an integer and 9<x^2<99, then what is the value of maximum possible value of x minus minimum possible value of x?
A. 5
B. 6
C. 7
D. 18
E. 20

Also tricky. Notice that \(x\) can take positive, as well as negative values to satisfy \(9<x^2<99\), hence \(x\) can be: -9, -8, -7, -6, -4, 4, 5, 6, 7, 8, or 9. We asked to find the value of \(x_{max}-x_{min}\), ans since \(x_{max}=9\) and \(x_{min}=-9\) then \(x_{max}-x_{min}=9-(-9)=18\).

Answer: D.



That is tricky. I took 3<x<10
So minimum value of x= 4
Maximum value of x= 9


Bunuel
When a question involves ^2 or any even power, then we have to consider the negative value?

Will that be the case in all situations?
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New post 11 Jul 2017, 04:50
Shiv2016 wrote:
Bunuel wrote:
2. If x is an integer and 9<x^2<99, then what is the value of maximum possible value of x minus minimum possible value of x?
A. 5
B. 6
C. 7
D. 18
E. 20

Also tricky. Notice that \(x\) can take positive, as well as negative values to satisfy \(9<x^2<99\), hence \(x\) can be: -9, -8, -7, -6, -4, 4, 5, 6, 7, 8, or 9. We asked to find the value of \(x_{max}-x_{min}\), ans since \(x_{max}=9\) and \(x_{min}=-9\) then \(x_{max}-x_{min}=9-(-9)=18\).

Answer: D.



That is tricky. I took 3<x<10
So minimum value of x= 4
Maximum value of x= 9


Bunuel
When a question involves ^2 or any even power, then we have to consider the negative value?

Will that be the case in all situations?


Yes, x^2 = (positive integer) has two solutions for x, positive and negative.
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New post 12 Jul 2017, 17:28
Bunuel wrote:
6. Anna has 10 marbles: 5 red, 2 blue, 2 green and 1 yellow. She wants to arrange all of them in a row so that no two adjacent marbles are of the same color and the first and the last marbles are of different colours. How many different arrangements are possible?
A. 30
B. 60
C. 120
D. 240
E. 480

Seems tough and complicated but if we read the stem carefully we find that the only way both conditions to be met for 5 red marbles, which are half of total marbles, they can be arranged only in two ways: R*R*R*R*R* or *R*R*R*R*R.

Here comes the next good news, in these cases BOTH conditions are met for all other marbles as well: no two adjacent marbles will be of the same color and the first and the last marbles will be of different colors.

Now, it's easy: 2 blue, 2 green and 1 yellow can be arranged in 5 empty slots in 5!/(2!*2!)=30 ways (permutation of 5 letters BBGGY out of which 2 B's and 2 G' are identical). Finally as there are two cases (R*R*R*R*R* and *R*R*R*R*R. ) then total # of arrangement is 30*2=60.

Answer: B.


Hi Bunuel,

Is this option possible: *R*R_R*R*R*? Or "in a row" means we can not have any space in between?
Thank you
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New post 12 Jul 2017, 21:26
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Soul777 wrote:
Bunuel wrote:
6. Anna has 10 marbles: 5 red, 2 blue, 2 green and 1 yellow. She wants to arrange all of them in a row so that no two adjacent marbles are of the same color and the first and the last marbles are of different colours. How many different arrangements are possible?
A. 30
B. 60
C. 120
D. 240
E. 480

Seems tough and complicated but if we read the stem carefully we find that the only way both conditions to be met for 5 red marbles, which are half of total marbles, they can be arranged only in two ways: R*R*R*R*R* or *R*R*R*R*R.

Here comes the next good news, in these cases BOTH conditions are met for all other marbles as well: no two adjacent marbles will be of the same color and the first and the last marbles will be of different colors.

Now, it's easy: 2 blue, 2 green and 1 yellow can be arranged in 5 empty slots in 5!/(2!*2!)=30 ways (permutation of 5 letters BBGGY out of which 2 B's and 2 G' are identical). Finally as there are two cases (R*R*R*R*R* and *R*R*R*R*R. ) then total # of arrangement is 30*2=60.

Answer: B.


Hi Bunuel,

Is this option possible: *R*R_R*R*R*? Or "in a row" means we can not have any space in between?
Thank you


In R*R*R*R*R* each * is a place for 2 blue, 2 green and 1 yellow marbles. So, * cannot accommodate say 2 marbles.
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New post 18 Aug 2017, 14:21
Bunuel wrote:
3. Fanny and Alexander are 360 miles apart and are traveling in a straight line toward each other at a constant rate of 25 mph and 65 mph respectively, how far apart will they be exactly 1.5 hours before they meet?
A. 25 miles
B. 65 miles
C. 70 miles
D. 90 miles
E. 135 miles

Make it simple! The question is: how far apart will they be exactly 1.5 hours before they meet? As Fanny and Alexander's combined rate is 25+65 mph then 1.5 hours before they meet they'll be (25+65)*1.5=135 miles apart.

Answer: E.


I couldn't understand (25+65)*1.5=135 miles apart i above solution. Why are we multiple combined rate with 1.5.
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New post 19 Aug 2017, 03:39
ammuseeru wrote:
Bunuel wrote:
3. Fanny and Alexander are 360 miles apart and are traveling in a straight line toward each other at a constant rate of 25 mph and 65 mph respectively, how far apart will they be exactly 1.5 hours before they meet?
A. 25 miles
B. 65 miles
C. 70 miles
D. 90 miles
E. 135 miles

Make it simple! The question is: how far apart will they be exactly 1.5 hours before they meet? As Fanny and Alexander's combined rate is 25+65 mph then 1.5 hours before they meet they'll be (25+65)*1.5=135 miles apart.

Answer: E.


I couldn't understand (25+65)*1.5=135 miles apart i above solution. Why are we multiple combined rate with 1.5.


Because 1.5 hours before they meet, the distance left to cover would be (combined rate)*(time) = (25+65)*1.5 = 135 miles.
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New post 17 Dec 2017, 08:09
The worst case scenario will be when 5 white chips are selected in a row. So the answer should be 6 (Option 'C').
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New post 17 Dec 2017, 09:08
prwkingdom wrote:
The worst case scenario will be when 5 white chips are selected in a row. So the answer should be 6 (Option 'C').


The correct answer to question 6 should be and is A, not C. Check here: https://gmatclub.com/forum/12-easy-piec ... l#p1033935

8. There are 15 black chips and 5 white chips in a jar. What is the least number of chips we should pick to guarantee that we have 2 chips of the same color?
A. 3
B. 5
C. 6
D. 16
E. 19

Worst case scenario would be if the first two chips we pick will be of the different colors. But the next chip must match with either of two, so 3 is the answer.

Answer: A.

Check other Worst Case Scenario Questions from our Special Questions Directory to understand the concept better.
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New post 26 Nov 2018, 18:43
Is this true tho?

The question specifically asks 1.5 hours BEFORE they meet. This would mean that (assuming they met in 4 hours. [25 x 4 = 100 + 65 x 4 = 260 TOTAL 360] The question asks about the 2.5th hour (4 hours - 1.5 hours), they would have been 100 miles apart at this juncture (25 x 2.5 = 62.50 + 65 x 2.5 = 162.5)

Bunuel wrote:
3. Fanny and Alexander are 360 miles apart and are traveling in a straight line toward each other at a constant rate of 25 mph and 65 mph respectively, how far apart will they be exactly 1.5 hours before they meet?
A. 25 miles
B. 65 miles
C. 70 miles
D. 90 miles
E. 135 miles

Make it simple! The question is: how far apart will they be exactly 1.5 hours before they meet? As Fanny and Alexander's combined rate is 25+65 mph then 1.5 hours before they meet they'll be (25+65)*1.5=135 miles apart.

Answer: E.
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Re: 12 Easy Pieces (or not?)  [#permalink]

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New post 28 Nov 2018, 01:02
Bunuel wrote:
8. There are 15 black chips and 5 white chips in a jar. What is the least number of chips we should pick to guarantee that we have 2 chips of the same color?
A. 3
B. 5
C. 6
D. 16
E. 19

Worst case scenario would be if the first two chips we pick will be of the different colors. But the next chip must match with either of two, so 3 is the answer.

Answer: A.


Hi Bunuel

coming up with the worst case is difficult for me.

Here i chose 2 scenarios:

1. worst case if i picked all 15 blacks, which gave me an answer of 17
2. worst case if i picked all 5 white, which gave and answer of 7

can you explain WHY picking one of each is the worst case?

regards
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New post 28 Nov 2018, 01:50
Hi Mansoor.

You're looking at the question the wrong way. You're going too far.

Here's what you do.

15 black + 5 white = 20 (total chips)

Odds of picking black 15/20 (3/4)

Odds of picking white 5/20 (1/4)

Now that you know this, think logically.

What do these ratios tell you.

3/4 and 1/4 (3:1).

They tell you that whenever you go out to pick from the jar, you'll pick 3 black chips for every 1 white chip. So, in order to guarantee that you have at least 2 chips of the same color, take out 3 chips and you guarantee the black color repeating at least once.

Make more sense?

Apologies, I'm typing this on my phone from work, excuse the grammar. PM me in case you still have questions.

Mansoor50 wrote:
Bunuel wrote:
8. There are 15 black chips and 5 white chips in a jar. What is the least number of chips we should pick to guarantee that we have 2 chips of the same color?
A. 3
B. 5
C. 6
D. 16
E. 19

Worst case scenario would be if the first two chips we pick will be of the different colors. But the next chip must match with either of two, so 3 is the answer.

Answer: A.


Hi Bunuel

coming up with the worst case is difficult for me.

Here i chose 2 scenarios:

1. worst case if i picked all 15 blacks, which gave me an answer of 17
2. worst case if i picked all 5 white, which gave and answer of 7

can you explain WHY picking one of each is the worst case?

regards


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New post 28 Nov 2018, 01:57
Just noticed a critical error in the way you're interpreting the question stem as well.

When it comes to these sorts of questions, unless specifically stated, assume the guy picking is blind as a bat. So the way you are approaching this question may be true in terms of a permutations/combinations scenario. (Mary is organizing her flavours as per black white black white... in 5 slots whatever). In this case, it helps to think of yourself as a dude who is blind as a bat.

What I would recommend is, before trying to read in to my previous comment, use this trick and analyse the question stem again. See what answer you come up with, again, you don't want to know how to solve this one specific question. You need a tool kit to approach any similar question the GMAT might throw at you.

All the best!

Mansoor50 wrote:

Bunuel wrote:
8. There are 15 black chips and 5 white chips in a jar. What is the least number of chips we should pick to guarantee that we have 2 chips of the same color?
A. 3
B. 5
C. 6
D. 16
E. 19

Worst case scenario would be if the first two chips we pick will be of the different colors. But the next chip must match with either of two, so 3 is the answer.

Answer: A.


Hi Bunuel

coming up with the worst case is difficult for me.

Here i chose 2 scenarios:

1. worst case if i picked all 15 blacks, which gave me an answer of 17
2. worst case if i picked all 5 white, which gave and answer of 7

can you explain WHY picking one of each is the worst case?

regards


Posted from my mobile device
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Re: 12 Easy Pieces (or not?)   [#permalink] 28 Nov 2018, 01:57

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