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An exam consists of 8 true/false questions. Brian forgets to

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An exam consists of 8 true/false questions. Brian forgets to [#permalink] New post 23 Jul 2014, 21:18
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An exam consists of 8 true/false questions. Brian forgets to study, so he must guess blindly on each question. If any score above 70% is a passing grade, what is the probability that Brian passes?

A) 1/16
B) 37/256
C) 1/2
D) 219/256
E) 15/16
[Reveal] Spoiler: OA
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Re: An exam consists of 8 true/false questions. Brian forgets to [#permalink] New post 23 Jul 2014, 21:34
maggie27 wrote:
An exam consists of 8 true/false questions. Brian forgets to study, so he must guess blindly on each question. If any score above 70% is a passing grade, what is the probability that Brian passes?

A) 1/16
B) 37/256
C) 1/2
D) 219/256
E) 15/16


Since 70% is the passing rate, .70 ( 8 questions) = 5.6 correct answers (or 6 rounded up)

We must then look for the probability of passing:
All correct answers + 7 correct answers + 6 correct answers = probability of passing.

All correct answers = \frac{1}{2}^8=\frac{1}{256}

7 correct answers = \frac{8C1}{1/2^8} = \frac{8}{256}

6 correct answers = \frac{8C2}{1/2^8} = \frac{28}{256}

\frac{1}{256}+\frac{8}{256}+\frac{28}{256}=\frac{37}{256}

Answer is B
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An exam consists of 8 true/false questions. Brian forgets to [#permalink] New post 24 Jul 2014, 07:49
maggie27 wrote:
An exam consists of 8 true/false questions. Brian forgets to study, so he must guess blindly on each question. If any score above 70% is a passing grade, what is the probability that Brian passes?

A) 1/16
B) 37/256
C) 1/2
D) 219/256
E) 15/16


I am not sure if the wording of the question is accurate. Anyone else feels the same way?

There are only two possibilities - Either Brian fails or he passes. So, probability that Brian passes = 1/2

Did I read the question incorrectly or is the wording ambiguous?
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An exam consists of 8 true/false questions. Brian forgets to [#permalink] New post 24 Jul 2014, 07:56
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Game wrote:
maggie27 wrote:
An exam consists of 8 true/false questions. Brian forgets to study, so he must guess blindly on each question. If any score above 70% is a passing grade, what is the probability that Brian passes?

A) 1/16
B) 37/256
C) 1/2
D) 219/256
E) 15/16


I am not sure if the wording of the question is accurate. Anyone else feels the same way?

There are only two possibilities - Either Brian fails or he passes. So, probability that Brian passes = 1/2

Did I read the question incorrectly or is the wording ambiguous?


No, that's not correct. According to your logic the probability of me dating Charlize Theron is also 1/2. Either yes or no. Wish this was true.
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Re: An exam consists of 8 true/false questions. Brian forgets to [#permalink] New post 24 Jul 2014, 17:20
Bunuel wrote:
Game wrote:
maggie27 wrote:
An exam consists of 8 true/false questions. Brian forgets to study, so he must guess blindly on each question. If any score above 70% is a passing grade, what is the probability that Brian passes?

A) 1/16
B) 37/256
C) 1/2
D) 219/256
E) 15/16


I am not sure if the wording of the question is accurate. Anyone else feels the same way?

There are only two possibilities - Either Brian fails or he passes. So, probability that Brian passes = 1/2

Did I read the question incorrectly or is the wording ambiguous?



No, that's not correct. According to your logic the probability of me dating Charlize Theron is also 1/2. Either yes or no. Wish this was true.


OK. Got it.
On a side note, probability of you dating Charlize Theron is 0 :) & here is the mathematical explanation for the math expert:

Assuming Charlize Theron (C.T.) will date only one person at a time
Probability(Game dating C.T.) + Probability(Bunuel dating C.T.) + ... = 1
Since Probability(Game dating C.T.) = 1
Probability(Bunuel dating C.T.) + ... = 1 - 1 = 0
:)
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An exam consists of 8 true/false questions. Brian forgets to [#permalink] New post 09 Aug 2014, 14:54
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An exam consists of 8 true/false questions. Brian forgets to study, so he must guess blindly on each question. If any score above 70% is a passing grade, what is the probability that Brian passes?

(A) 1/16
(B) 37/256
(C) 5/32
(D) 219/256
(E) 15/16

Could anyone please explain?
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Re: An exam consists of 8 true/false questions. Brian forgets to [#permalink] New post 09 Aug 2014, 16:02
An exam consists of 8 true/false questions. Brian forgets to study, so he must guess blindly on each question. If any score above 70% is a passing grade, what is the probability that Brian passes?

(A) 1/16
(B) 37/256
(C) 5/32
(D) 219/256
(E) 15/16

Could anyone please explain?


If you have 8 T or F and Brian is going to guess then each question he has a 50% chance of getting correct.

If a passing score is 70% it means Brian needs to get 6/8=75%, 7/8=87.5%, or 8/8=100% to pass. Each is a possibility. If Brian gets a 5/8(=62.5%) or below he fails.

So first figure out the number of ways that Brian can get 6 out of 8, 7 out of 8, and 8 out of 8 questions correct.
Which is 8 choose 6, equals is 28, 8 choose 7, equals 8, and 8 choose 8, equals 1. This sums to 37.

The number of possible questions outcomes -the sum of 8 choose 8, 7 choose 8, 6 choose 8….2 choose 8, 1 choose 8, and 0 choose 8 is 256, so the chance of him passing is 37/256.

I know there is a shorter way, but this is how I did it in about 2:34
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Re: An exam consists of 8 true/false questions. Brian forgets to [#permalink] New post 10 Aug 2014, 00:35
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If you have True or False question, then each question has a \frac{1}{2} chance of getting correct.

If a passing score is 70% it means Brian needs to get right or 6 questions (6/8=75%), or 7 questions (7/8=87.5%), or 8 questions (8/8=100%).

The probability to have all 8 questions right is: \frac{1}{2^8}
The probability to have 7 questions right and 1 wrong is: C_8^1*\frac{1}{2^7}*\frac{1}{2}=\frac{8}{2^8} (choose this incorrect answer C_8^1, probability to have 7 right \frac{1}{2^7}, and probability to have 1 wrong \frac{1}{2})
The probability to have 6 questions right and 2 wrong is: C_8^2*\frac{1}{2^6}*\frac{1}{2^2}=\frac{28}{2^8} (choose the se 2incorrect answers C_8^2, probability to have 6 right \frac{1}{2^6}, and probability to have 2 wrong \frac{1}{2^2})

The probability is \frac{1}{2^8}+\frac{8}{2^8}+\frac{28}{2^8}=\frac{37}{256}
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Re: An exam consists of 8 true/false questions. Brian forgets to [#permalink] New post 12 Aug 2014, 08:15
Expert's post
Sen1120 wrote:
An exam consists of 8 true/false questions. Brian forgets to study, so he must guess blindly on each question. If any score above 70% is a passing grade, what is the probability that Brian passes?

(A) 1/16
(B) 37/256
(C) 5/32
(D) 219/256
(E) 15/16

Could anyone please explain?


Merging similar topics. Please refer to the discussion above.
_________________

NEW TO MATH FORUM? PLEASE READ THIS: ALL YOU NEED FOR QUANT!!!

PLEASE READ AND FOLLOW: 11 Rules for Posting!!!

RESOURCES: [GMAT MATH BOOK]; 1. Triangles; 2. Polygons; 3. Coordinate Geometry; 4. Factorials; 5. Circles; 6. Number Theory; 7. Remainders; 8. Overlapping Sets; 9. PDF of Math Book; 10. Remainders; 11. GMAT Prep Software Analysis NEW!!!; 12. SEVEN SAMURAI OF 2012 (BEST DISCUSSIONS) NEW!!!; 12. Tricky questions from previous years. NEW!!!;

COLLECTION OF QUESTIONS:
PS: 1. Tough and Tricky questions; 2. Hard questions; 3. Hard questions part 2; 4. Standard deviation; 5. Tough Problem Solving Questions With Solutions; 6. Probability and Combinations Questions With Solutions; 7 Tough and tricky exponents and roots questions; 8 12 Easy Pieces (or not?); 9 Bakers' Dozen; 10 Algebra set. ,11 Mixed Questions, 12 Fresh Meat

DS: 1. DS tough questions; 2. DS tough questions part 2; 3. DS tough questions part 3; 4. DS Standard deviation; 5. Inequalities; 6. 700+ GMAT Data Sufficiency Questions With Explanations; 7 Tough and tricky exponents and roots questions; 8 The Discreet Charm of the DS ; 9 Devil's Dozen!!!; 10 Number Properties set., 11 New DS set.


What are GMAT Club Tests?
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Re: An exam consists of 8 true/false questions. Brian forgets to   [#permalink] 12 Aug 2014, 08:15
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