Ask me Anything about GMAT Quant and Math in General Hello and welcome to my PS Forum Expert Topic . Let me know if you have any Problem Solving questions I can help with - strategy, specific ones, etc. You may know me from forum discussion - I am one of the resident Veritas Prep Experts on GMAT Club. I have over 8,000 posts and 22,000 Kudos. I hope you find my replies and explanations helpful. My most helpful reply to this OG Question received over 100 Kudos. If you have any specific questions about PS or Math in general such as strategy, approach, etc, please feel free to post them here and I will be happy to address them. I intend to have this thread be as a "Everything You Need to Know about PS" type of thread and will include some FAQ's over time. Thank you all - good luck on the GMAT and look forward to seeing you in the PS forum! Karishma.

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Veritas Prep PS Forum Expert - Karishma - Ask Me Anything about Math

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KarishmaB

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27 May 2019, 22:44

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indu1954

*(probability) 4 traffic lights A, B, C, D have red light. If the probabilities that the red lights of A, B, C, and D hold are 0.5, 0.6, 0.7, and 0.8, respectively, what is the probability that a car passes by A and D and stops B and C, approximately?
A. 4% B. 7% C. 10% D. 15% E. 40%

Looking at the screenshot you sent, it looks like the question is exactly as you have written. I would suggest you to ignore this source since it seems to be very sub-standard.

As for this question, if the red light holds, the car stops. If it changes, the car passes through.

Probability that A and D turn while B and C hold = 0.5 * 0.6 * 0.7 * 0.2 = 4.2%

Sandeepanisha

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06 Jun 2019, 14:24

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Hi Karishma,
(geometry) Two people run the circle with 10 rounds, one runs in the inside and the other runs in the outside. When the difference between the inside and the outside is 15 feet, what is the difference of between two people's distance after running 10 rounds, approximately?
A. 800 B. 850 C. 900 D. 950 E. 1,000

ajaysharma121

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09 Jun 2019, 11:01

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hi Karishma,
Please help me to solve this
https://gmatclub.com/forum/as-the-pictu ... 31845.html

indu1954

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11 Jun 2019, 11:14

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Need help to Solve the quant Question.

https://gmatclub.com/forum/geometry-297755.html

KarishmaB

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12 Jun 2019, 00:25

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Sandeepanisha

I don't understand this question. It seems to say that one person runs 10 rounds outside and another person runs 10 rounds inside. If the difference in the circumference is 15 feet, the one outside will run 10*15 feet extra. If the difference of 15 feet is between radii or diameter, it will be different. The question needs to clarify properly.

KarishmaB

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12 Jun 2019, 00:26

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ajaysharma121

hi Karishma,
Please help me to solve this
https://gmatclub.com/forum/as-the-pictu ... 31845.html

Hey ajaysharma121,

This link is broken. Can you please repost it?

KarishmaB

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12 Jun 2019, 00:27

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indu1954

Need help to Solve the quant Question.

https://gmatclub.com/forum/geometry-297755.html

Done.

indu1954

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12 Jun 2019, 18:50

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Hi Karishma can you please help me with this?
https://gmatclub.com/forum/geometry-297753.html

KarishmaB

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13 Jun 2019, 03:18

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indu1954

Hi Karishma can you please help me with this?
https://gmatclub.com/forum/geometry-297753.html

Hey indu1954,

The requested post doesn't exist anymore. Could you please check!

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13 Jun 2019, 03:26

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Hi Karishma,

Weighted average is a gray area section for me, can you please help me in this problem.

https://gmatclub.com/forum/1-unit-of-x- ... 96986.html

Posted from my mobile device

KarishmaB

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13 Jun 2019, 03:55

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Shef08

Hi Karishma,

Weighted average is a gray area section for me, can you please help me in this problem.

https://gmatclub.com/forum/1-unit-of-x- ... 96986.html

Posted from my mobile device

Hey Shef08,

Done here: https://gmatclub.com/forum/1-unit-of-x- ... l#p2292835

If you struggle with weighted averages, check out this post: https://www.gmatclub.com/forum/veritas-prep-resource-links-no-longer-available-399979.html#/2011/0 ... -averages/

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16 Jun 2019, 00:13

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Hi VeritasKarishma, Can you please tell me most important quant topics I need to cover. I have a booked GMAT next week. I thought I was good at quant so I concentrated mostly on my verbal ability. Now my mock score is only 45. My target is 50/51. I am ready to put in 100 hours this week.Thanks!

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16 Jun 2019, 23:00

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GittinGud

Number Properties, Algebra and Geometry are the most important topics. Also, ensure that you understand the Data Sufficiency format really well. What is sufficiency, how to avoid common traps etc.

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01 Aug 2019, 20:39

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Hi VeritasKarishma

I had a doubt and its not clear to me yet.

If xyz ≠ 0, xy > 0, and x^2 + xy + xz < 0, then which of the following must be true?

I. x(y + z) < 0

II. x + y + z < 0

III. If x < 0, then z > 0.

A) I only
B) I and II only
C) I and III only
D) II and III only
E) I, II, and III

Can you please help me simplify why it cannot be E

KarishmaB

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01 Aug 2019, 21:15

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Nums99

xyz ≠ 0 implies none of x, y and z is 0.

xy > 0 implies both x and y have the same sign. Either both are positive or both negative.

x^2 + xy + xz < 0 implies xz is negative so x and z have opposite signs. x^2 is positive because squares cannot be negative. xy is positive we have already benn given so xz must be negative.

I. x(y + z) < 0

We know that the expression x^2 + xy + xz is negative
x^2 must be positive. So if we remove x^2 from above, the expression (xy + xz) will become even more negative.
Hence, this must be true.

II. x + y + z < 0
x^2 + xy + xz < 0
x(x + y + z) < 0
If x is positive, (x+y+z) < 0
If x is negative, (x + y + z) > 0 (Take x = -3, y = -1 and z = 100 for example)
So this is not necessarily true.

III. If x < 0, then z > 0.
Correct. x and z have opposite signs. So this must be true.

Answer (C)

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09 Aug 2019, 06:02

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hi VeritasKarishma

The product of 3 consecutive positive multiples of 4 must be divisible by each of the following EXCEPT:

A)16
B)36
C)48
D)128
E)192

The OA is B but i don't understand why exactly.
Arent all these numbers divisible by three consecutive multiples of 4

KarishmaB

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13 Aug 2019, 21:41

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Nums99

Say the 3 consecutive multiples are 4, 8 and 12. Their product = 4*8*12. This product is not divisible by 36.
36 = 4*9
The product does not have 9 in it (two 3s). It necessarily will have one 3 but it may or may not have two 3s. Hence it may not be divisible by 36.

Three consecutive multiples of 4 will have three 4s, another 2 and a 3 necessarily. So the product will be divisible by 2^7 * 3 and all its factors.

For more on this, check:
https://www.gmatclub.com/forum/veritas-prep-resource-links-no-longer-available-399979.html#/2011/0 ... c-or-math/
https://www.gmatclub.com/forum/veritas-prep-resource-links-no-longer-available-399979.html#/2011/0 ... h-part-ii/

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25 Aug 2019, 04:38

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Hiii VeritasKarishma

https://gmatclub.com/forum/if-6-4-x-5-w ... 99799.html

Referring to the question on this page

i am really confused when they say " |x|=-x " isn't the only possibility to this that x = 0???

because no number inside a mod sign can be negative as such

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25 Aug 2019, 19:34

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VeritasKarishma

tilak123 - I am kicking off this post as per your request.

This is a compilation of all links to the "Made Easy" series. Bunuel has already compiled links to all my posts on specific topics. I will provide links to those compilations:

Statistics Made Easy

Inequalities Made Easy

Sequences Made Easy

Combinatorics Made Easy

Weighted Avgs and Mixtures Made Easy

Hi Karishma.. I request reference to Question 1 in your post "A group of 8 friends sit together in a circle. If A refuses to sit beside B unless C sits on the other side of A as well, how many possible seating arrangements are possible?" Even though i understood your explanation, can u please tell me whats wrong in my approach?

My Approach: There are 2 scenarios possible, One where A sits next to both B & C and the other wherein A doesn't sit next to both B & C . In the former the no.of arrangements are: 1*2*1*5*4*3*2*1= 2*5!
In the latter the no. of arrangements are: 1*5*4*5*4*3*2*1= 20*5!
Thus total no. of arrangements are: 5!(20+2)= 22*5!
However your answer is 32*5!

Where am i missing??

KarishmaB

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25 Aug 2019, 21:12

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Nums99

Your assumption is that x is a positive number and hence, -x is a negative number. But can -x be a POSITIVE number? Think about it.

What about the case when x = -5? Then -x = - (-5) = 5

What about the case when x = -23? Then -x = - (-23) = 23

-x can be positive when x is negative. In these cases, -x = |x|

And hence, we define |x| as:

|x| = x when x >= 0
|x| = -x when x < 0