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# The data in the table show cell phone ownership status by grade level

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Math Expert
Joined: 02 Sep 2009
Posts: 58327
The data in the table show cell phone ownership status by grade level  [#permalink]

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23 Mar 2018, 00:00
00:00

Difficulty:

75% (hard)

Question Stats:

27% (01:37) correct 73% (01:26) wrong based on 63 sessions

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The data in the table show cell phone ownership status by grade level of 300 middle school students. If one of the 300 students is randomly selected, what is the probability that the student owns a cell phone, given that the student is a seventh grader?

A. 11/60

B. 11/100

C. 163/300

D. 55/163

E. 11/20

Attachment:

2018-03-23_1057.png [ 46.14 KiB | Viewed 970 times ]

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Current Student
Joined: 07 Jan 2016
Posts: 1088
Location: India
GMAT 1: 710 Q49 V36
Re: The data in the table show cell phone ownership status by grade level  [#permalink]

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23 Mar 2018, 00:17
Bunuel wrote:
The data in the table show cell phone ownership status by grade level of 300 middle school students. If one of the 300 students is randomly selected, what is the probability that the student owns a cell phone, given that the student is a seventh grader?

A. 11/60

B. 11/100

C. 163/300

D. 55/163

E. 11/20

Attachment:
2018-03-23_1057.png

total students 300

7th grader with cellphone 55

prob = 55/300 = 11/60
Manager
Joined: 28 Jan 2018
Posts: 50
Location: Netherlands
Concentration: Finance
GMAT 1: 710 Q50 V36
GPA: 3
Re: The data in the table show cell phone ownership status by grade level  [#permalink]

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23 Mar 2018, 10:21
Bunuel wrote:
The data in the table show cell phone ownership status by grade level of 300 middle school students. If one of the 300 students is randomly selected, what is the probability that the student owns a cell phone, given that the student is a seventh grader?

A. 11/60

B. 11/100

C. 163/300

D. 55/163

E. 11/20

Attachment:
2018-03-23_1057.png

The probability that the random selected student will be a 7th grader and own a cell phone will be: $$\frac{55}{300} = \frac{11}{60}$$ (A)
Intern
Joined: 26 Jun 2014
Posts: 20
Re: The data in the table show cell phone ownership status by grade level  [#permalink]

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23 Mar 2018, 13:15
Bunuel wrote:
The data in the table show cell phone ownership status by grade level of 300 middle school students. If one of the 300 students is randomly selected, what is the probability that the student owns a cell phone, given that the student is a seventh grader?

A. 11/60

B. 11/100

C. 163/300

D. 55/163

E. 11/20

The question says "GIVEN that the student is in the 7th grade". That means we can ignore the rest of the chart and only look at the 7th grade row. 55/100 is 11/20, answer E.
Manager
Joined: 28 Jan 2018
Posts: 50
Location: Netherlands
Concentration: Finance
GMAT 1: 710 Q50 V36
GPA: 3
Re: The data in the table show cell phone ownership status by grade level  [#permalink]

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23 Mar 2018, 14:09
Chakolate wrote:
Bunuel wrote:
The data in the table show cell phone ownership status by grade level of 300 middle school students. If one of the 300 students is randomly selected, what is the probability that the student owns a cell phone, given that the student is a seventh grader?

A. 11/60

B. 11/100

C. 163/300

D. 55/163

E. 11/20

The question says "GIVEN that the student is in the 7th grade". That means we can ignore the rest of the chart and only look at the 7th grade row. 55/100 is 11/20, answer E.

We're asked to choose 1 student from the total of 300, so your answer is wrong!
Intern
Joined: 26 Jun 2014
Posts: 20
Re: The data in the table show cell phone ownership status by grade level  [#permalink]

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23 Mar 2018, 19:23
hoang221 wrote:

We're asked to choose 1 student from the total of 300, so your answer is wrong!

You say I'm wrong, I say you're wrong. I guess we'll have to wait until Bunuel weighs in.
Intern
Joined: 15 Feb 2019
Posts: 4
GMAT 1: 690 Q49 V34
GMAT 2: 690 Q47 V38
GMAT 3: 710 Q50 V36
The data in the table show cell phone ownership status by grade level  [#permalink]

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07 Jul 2019, 08:38
Use Bayes' theorem. P(A|B) = P(B|A)P(A)/P(B). We have:

P(A|B) is the prob that given a student in 7th grade, he/she has a smartphone (what we want to find).

P(B|A) is the prob. that given a student in 7th grade with a smartphone, he/she is in the 7th grade. Therefore, P(B|A) = 1.

P(A) is the prob. that a 7th grade student has a smartphone. Hence, P(A) = 55/300.

P(B) is the prob. that a student is in 7th grade. Thus, P(B) = 1/3.

Finally, P(A|B) = 1(55/300)/3 = 165/300 = 11/20.

Hope this helps.
The data in the table show cell phone ownership status by grade level   [#permalink] 07 Jul 2019, 08:38
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# The data in the table show cell phone ownership status by grade level

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