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examPAL DS Forum Expert David - Ask Me Anything about GMAT DS

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examPAL Representative
Joined: 07 Dec 2017
Posts: 1154

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10 Jun 2019, 02:06
1
indu1954 wrote:
(statistics) 5 points v, w, x, y, and z are on the number line. Is the median of
them equal to their average (arithmetic mean)?
1) v<w<x<y<z
2) z-y=y-x=x-w=w-v

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Joined: 02 Nov 2017
Posts: 32
Location: India
GPA: 3.87

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10 Jul 2019, 06:56
Jeonghee has 5 different red cards and 5 different blue cards. She shuffles the10 cards, and then places 5 of the cards in a row. What is the probability that all red cards are adjacent to each other and all blue cards are adjacent to each other in her row?

A. 2/5
B. 28/125
C. 31/126
D. 33/140
E. 25/216

I didn't understand the solution so please elaborate it
examPAL Representative
Joined: 07 Dec 2017
Posts: 1154

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21 Aug 2019, 03:48
1
indu1954 wrote:
Jeonghee has 5 different red cards and 5 different blue cards. She shuffles the10 cards, and then places 5 of the cards in a row. What is the probability that all red cards are adjacent to each other and all blue cards are adjacent to each other in her row?

A. 2/5
B. 28/125
C. 31/126
D. 33/140
E. 25/216

I didn't understand the solution so please elaborate it

Hello indu1954,
Apologies for the delay in response, if you want me to address a question quickly please tag me in it.

This is a difficult question. The most straightforward (easy-to-understand) solution is to split into cases.

All 5 cards have the same color:
Pick all the blue cards (5C5 = 1) and arrange them (5!)
Pick all the red cards (5C5 = 1) and arrange them (5!)
Total 5! + 5! = 240 ways

Ratio is 1:4
Pick 1 blue card (5C1 = 5) and 'arrange it' (1! = 1). Pick 4 red cards (5C4 = 5) and arrange them (4!). Choose if the order is blue --> red or red --> blue (2 ways).
Do the same for 1 red, 4 blue.
Total 5*5*4!*2 + 5*1!*5*4!*2 = 100*4! = 2400 ways.

Ratio is 2:3
Pick 2 blue cards (5C2 = 10) and arrange them (2! = 2). Pick 3 red cards (5C3 = 10) and arrange them (3!). Choose if the order is blue --> red or red --> blue (2 ways).
Do the same for 2 red, 3 blue.
Total 10*2*10*6*2 times 2 = 200*4! = 4800 ways.

So there are 4800 + 2400 + 240 = 7440 ways in total
The total number of ways to pick 5 cards out of 10 and arrange them is 10C5 * 5! = 10*9*8*7*6

Then the answer is 7440/10*9*8*7*6 = 744/9*8*7*6 = (800 - 56)/9*8*7*6 = (100 - 7)/9*7*6 = 93/9*7*6 = 31/3*7*6= 31/126.
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Joined: 19 Oct 2019
Posts: 8

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03 Nov 2019, 22:45
Is x>0 ,
1)|3 - x|< |x + 5| (2) |3 - 2x| < x - 1
A) Statement (1) ALONE is sufficient, but statement (2) alone is not sufficient.
B)Statement (2) ALONE is sufficient, but statement (1) alone is not sufficient.
C)BOTH statements TOGETHER are sufficient, but NEITHER statement ALONE is sufficient.
D)EACH statement ALONE is sufficient.
E)Statements (1) and (2) TOGETHER are NOT sufficient.

I am not able to solve this kind of absolute value questions. I need an algebraic approach to solve such kind of questions..

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Joined: 09 Aug 2019
Posts: 8

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07 Nov 2019, 21:14
Hello david,
How to solve the DS questions in under 2 minutes? What should be the hit rate if one were to get 80%le + score in DS
Intern
Joined: 22 Feb 2020
Posts: 2

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22 Feb 2020, 14:36
indu1954 wrote:
(ex 1) A certain store, books are sold. Books are hard cover or soft cover and hard cover books sold \$10 each and soft cover books sold \$6. Is the number of hard cover books sold greater than that of soft cover books sold?
1) The average price sold of total books is \$9
2) The number of hard cover books sold is 100

Figure out what sort of information you need to answer the given DS question BEFORE you look at the given statements. If you can do this (and in most questions you can), it will provide you with enormous focus when analyzing the given statements and is very useful for avoiding various 'traps' that the GMAT loves to use.
Re: examPAL DS Forum Expert David - Ask Me Anything about GMAT DS   [#permalink] 22 Feb 2020, 14:36

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