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Bunuel
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[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
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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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?
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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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