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Two girls have their birthdays in the same week. It is known that one

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Bunuel
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Two girls have their birthdays in the same week. It is known that one [#permalink] New post   28 Jan 2021, 05:05
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12% (01:07) correct 88% (01:09) wrong based on 69 sessions
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Two girls have their birthdays in the same week. It is known that one of the girls was born on a Saturday, what is the probability that both were born on a Saturday?

A. 1/2
B. 1/4
C. 1/7
D. 1/13
E. 1/14

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D
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Re: Two girls have their birthdays in the same week. It is known that one [#permalink] New post   08 Feb 2021, 12:16
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Bunuel wrote:
Two girls have their birthdays in the same week. It is known that one of the girls was born on a Saturday, what is the probability that both were born on a Saturday?

A. 1/2
B. 1/4
C. 1/7
D. 1/13
E. 1/14

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Solution:

Since we already know that one of the girls was born on a Saturday (probability = 1), the probability that the other girl was born on a Saturday is 1/7. Thus, the probability that both were born on Saturday is 1 x 1/7 = 1/7.

Answer: C
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CEdward
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Re: Two girls have their birthdays in the same week. It is known that one [#permalink] New post   13 Feb 2021, 16:28
We already know the first girl was born on Saturday. So the question is, what's the probability that the 2nd girl will be born on Saturday? 1/7

Answer is C.

There is something glitchy with the choices though.
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Re: Two girls have their birthdays in the same week. It is known that one [#permalink] New post   13 Feb 2021, 17:18
[quote="Bunuel"]Two girls have their birthdays in the same week. It is known that one of the girls was born on a Saturday, what is the probability that both were born on a Saturday?

A. 1/2
B. 1/4
C. 1/7
D. 1/13
E. 1/14


Bunuel can you please explain why the answer is 'D' instead of 'C'..!
P(both girls having their birthdays same week)= P(first girls b'day on sat)*P(Second girls b'day on sat)
= 1* (1/7) =1/7
am I missing any thing.?
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Bunuel
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Re: Two girls have their birthdays in the same week. It is known that one [#permalink] New post   14 Feb 2021, 03:02
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Bunuel wrote:
Two girls have their birthdays in the same week. It is known that one of the girls was born on a Saturday, what is the probability that both were born on a Saturday?

A. 1/2
B. 1/4
C. 1/7
D. 1/13
E. 1/14

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Anyone else want to try this one?
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Two girls have their birthdays in the same week. It is known that one [#permalink] New post   Updated on: 17 Feb 2021, 09:32
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I got the answer to be D 1/13

Given the condition that one of the girls was born on a Saturday, all possible situations will amount to 13/49 (1/7*6/7 + 1/7*6/7 + 1/7*1/7). Then the one favourable case is 1/49.

If we divide the later by the former, we will get 1/13.

Am I correct in doing this?

Originally posted by Pratick11 on 14 Feb 2021, 05:17.
Last edited by Pratick11 on 17 Feb 2021, 09:32, edited 1 time in total.
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Re: Two girls have their birthdays in the same week. It is known that one [#permalink] New post   14 Feb 2021, 17:50
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On the first try I got this wrong, but then I understood the underlying assumption and the way to work the problem out: If the first girl is known to have her birthday on Saturday, the possible options are:

Birthday Girl 1 & Birthday Girl 2

1. Saturday & Saturday
2. Saturday & Sunday
3. Saturday & Monday
4. Saturday & Tuesday
5. Saturday & Wednesday
6. Saturday & Thursday
7. Saturday & Friday
8. Monday & Saturday
9. Tuesday & Saturday
10. Wednesday & Saturday
11. Thursday & Saturday
12. Friday & Saturday
13. Sunday & Saturday

Therefore you have only outcome number 1 as the favorable option, over all the possible combinations that are 13.

Answer choice D: 1/13.
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Re: Two girls have their birthdays in the same week. It is known that one [#permalink]
14 Feb 2021, 17:50
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