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Explanation

by MalcolmW » Wed Nov 04, 2015 7:49 pm
Can anyone explain to me why the answer is A

In a certain class, one student is to be selected at random to read. What is the probability that a boy will read?
(1) Two-thirds of the students in the class are boys.
(2) Ten of the students in the class are girls.



My thought is that the correct answer would be C
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by theCEO » Wed Nov 04, 2015 8:37 pm
MalcolmW wrote:Can anyone explain to me why the answer is A

In a certain class, one student is to be selected at random to read. What is the probability that a boy will read?
(1) Two-thirds of the students in the class are boys.
(2) Ten of the students in the class are girls.

My thought is that the correct answer would be C
If one student is to be selected, the probability a boy will read = sum of all boys / total students
Therefore we want to know what is total boys/total students

1) 2/3 of students = boys
1/3 of students = girls
3/3 of students = boys + girls

total boys / total students = 2/3 of students / 3/3 of students = 2/3
Probability = 2/3
statement is sufficent

2)Ten of the students in the class are girls
We don't know whow many boys are present so we cannot calculate the probability of boys
statement is insufficent

ans = a
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by manik11 » Thu Nov 05, 2015 1:29 am
MalcolmW wrote:Can anyone explain to me why the answer is A

In a certain class, one student is to be selected at random to read. What is the probability that a boy will read?
(1) Two-thirds of the students in the class are boys.
(2) Ten of the students in the class are girls.



My thought is that the correct answer would be C
Let T = Total number of students in the class
Let B = Total number of boys in the class
Probability that a boy will be selected = B/T

(1) Two-thirds of the students in the class are boys.

Implies (2/3)T = B --> B/T = 2/3 --> SUFFICIENT

(2) Ten of the students in the class are girls.

The number of boys is not known.
There could be 10 boys for a total of 20 students in which case
B/T = 10/20 = 1/2

There could be 20 boys for a total of 30 students in which case
B/T = 20/30 = 2/3

Two different answers hence INSUFFICIENT

Final Answer : A
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by MalcolmW » Thu Nov 05, 2015 10:45 am
Thanks! I understand how you come the the conclusion of answer A.

My question is, how do you know how or what to think in this terms? I simply thought that bc they only told how many boys were in the class that there was not enough.
information.
I guess what I'm saying is a method of attack to see what the problem offers.
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by theCEO » Thu Nov 05, 2015 10:53 am
MalcolmW wrote:Thanks! I understand how you come the the conclusion of answer A.

My question is, how do you know how or what to think in this terms? I simply thought that bc they only told how many boys were in the class that there was not enough.
information.
I guess what I'm saying is a method of attack to see what the problem offers.
Because probability is a fraction and we are told the fraction of boys, the statement is sufficient.

The only way this statement would not be sufficient is we have more than 2 genders!

Does this make sense?
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by MalcolmW » Thu Nov 05, 2015 11:24 am
Unfortunately, no. I believe I'm having trouble approaching each problem the proper way.
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by theCEO » Fri Nov 06, 2015 2:18 am
MalcolmW wrote:Unfortunately, no. I believe I'm having trouble approaching each problem the proper way.
Ok. To help you out, please explain in details step by by step why you thought C was correct and then I will see where you went wrong.
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by [email protected] » Fri Nov 06, 2015 10:18 am
Hi MalcolmW,

DS questions 'test' a variety of skills besides basic math skills, including critical thinking, thoroughness, attention to detail, pattern-matching, the ability to prove that you're correct, etc.

While there are common 'steps' that you can go through while dealing with all DS questions, you have to pay attention to the information that you're given and what the questions specifically ASKS you for.

In this prompt, we're asked for the probability that a boy will be selected to read. To answer this question, there are a number of types of information that will give us what we need to know:

1) The exact number of boys and girls in the class.
2) The ratio of boys to girls
3) The ratio of boys to total students
4) The ratio of girls to total students

Each of the above options will give us enough information to answer the question, but if you don't consider some of the possibilities, then you might select one of the wrong answers.

1) How long have you been studying for the GMAT?
2) What resources have you been using?

GMAT assassins aren't born, they're made,
Rich
Contact Rich at [email protected]
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by MalcolmW » Fri Nov 06, 2015 11:28 am
@TheCeo

After reading the question I notice that the first statement gives me only a portion of what I need to completely answer the question. To my thinking, without the knowledge of how many girls are in the class I cannot properly answer the target question. Making it insufficient. I move on to the second statement and see that they give the opposite. Which lead me to think that together, I would be able to formulate somehow the chamces of a boy being picked.

My main worry is that I will end up trying to solve each question when I know that is not the purpose of DS.
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by MalcolmW » Sat Nov 07, 2015 4:47 am
theCEO wrote:
MalcolmW wrote:Unfortunately, no. I believe I'm having trouble approaching each problem the proper way.
Ok. To help you out, please explain in details step by by step why you thought C was correct and then I will see where you went wrong.
After reading the question I notice that the first statement gives me only a portion of what I need to completely answer the question. To my thinking, without the knowledge of how many girls are in the class I cannot properly answer the target question. Making it insufficient. I move on to the second statement and see that they give the opposite. Which lead me to think that together, I would be able to formulate somehow the chamces of a boy being picked.

My main worry is that I will end up trying to solve each question when I know that is not the purpose of DS.
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by theCEO » Sat Nov 07, 2015 6:03 am
theCEO wrote:
MalcolmW wrote:Unfortunately, no. I believe I'm having trouble approaching each problem the proper way.
Ok. To help you out, please explain in details step by by step why you thought C was correct and then I will see where you went wrong.
After reading the question I notice that the first statement gives me only a portion of what I need to completely answer the question.
As Rich pointed out above, there are a number of information that we need to answer the question. Since you think that this statement is missing information, it makes me come to the following conclusion that is based on my past behaviour.
1) You may be rushing to answer the question, and overlooking certain key words.
2) You may be familiar with similar type questions which produce an answer of "C", so your mind quickly jumps into making a judgement.
Trust me, we all made these mistakes at one time.

The aim of the GMAT is not for everyone to get a score of 800! Therefore treat all questions as if they are tricky. Use all the information that is given.
To my thinking, without the knowledge of how many girls are in the class I cannot properly answer the target question. Making it insufficient.
How would you solve the following question?

In a certain class, one student is to be selected at random to read. What is the probability that a boy will read?
(1) Two-thirds of the students in the class are boys and there are 10 girls in the class.
(2) Ten of the students in the class are girls.
I move on to the second statement and see that they give the opposite. Which lead me to think that together, I would be able to formulate somehow the chamces of a boy being picked.
This seems like you may be rushing to get to the answer and I now see what you are doing.
My main worry is that I will end up trying to solve each question when I know that is not the purpose of DS
What do you mean by this statement?
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by MalcolmW » Sat Nov 07, 2015 6:31 am
theCEO wrote:
theCEO wrote:
MalcolmW wrote:Unfortunately, no. I believe I'm having trouble approaching each problem the proper way.
Ok. To help you out, please explain in details step by by step why you thought C was correct and then I will see where you went wrong.
After reading the question I notice that the first statement gives me only a portion of what I need to completely answer the question.
As Rich pointed out above, there are a number of information that we need to answer the question. Since you think that this statement is missing information, it makes me come to the following conclusion that is based on my past behaviour.
1) You may be rushing to answer the question, and overlooking certain key words.
2) You may be familiar with similar type questions which produce an answer of "C", so your mind quickly jumps into making a judgement.
Trust me, we all made these mistakes at one time.

The aim of the GMAT is not for everyone to get a score of 800! Therefore treat all questions as if they are tricky. Use all the information that is given.
To my thinking, without the knowledge of how many girls are in the class I cannot properly answer the target question. Making it insufficient.
How would you solve the following question?

In a certain class, one student is to be selected at random to read. What is the probability that a boy will read?
(1) Two-thirds of the students in the class are boys and there are 10 girls in the class.
(2) Ten of the students in the class are girls.
I move on to the second statement and see that they give the opposite. Which lead me to think that together, I would be able to formulate somehow the chamces of a boy being picked.
This seems like you may be rushing to get to the answer and I now see what you are doing.
My main worry is that I will end up trying to solve each question when I know that is not the purpose of DS
What do you mean by this statement?
From my studies many references state that the main goal is to determine whether or not the statement gives enough to answer the target question. I think between this information and knowing that I cannot take 5min per question, I end up making mistakes.
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