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# [# 22] 2 Min. Challenge : Birthday

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CEO
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[# 22] 2 Min. Challenge : Birthday [#permalink]

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15 Mar 2004, 14:47
This topic is locked. If you want to discuss this question please re-post it in the respective forum.

Rules

1. Time yourself
2. Solve seperately

Two girls have their birthdays in the same Week. It is known that one
of the girls was born on a Saturday. Determine the probability that both
were born on a Saturday.

1/2
1/4
1/7
1/13
1/14
Senior Manager
Joined: 23 Sep 2003
Posts: 293
Location: US
Followers: 1

Kudos [?]: 4 [0], given: 0

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15 Mar 2004, 15:18
C - 1/7 - probability that second girl is born on a saturday.
Founder
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Posts: 14911
Location: United States (WA)
GMAT 1: 750 Q49 V42
GPA: 3.5
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Kudos [?]: 25096 [0], given: 4746

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15 Mar 2004, 16:11
Geethu wrote:
C) 1/7

<1 min

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15 Mar 2004, 17:58
I will go with 1/7

Few seconds.
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Re: [# 22] 2 Min. Challenge : Birthday [#permalink]

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15 Mar 2004, 18:03
praetorian123 wrote:
Rules

1. Time yourself
2. Solve seperately

Two girls have their birthdays in the same Week. It is known that one
of the girls was born on a Saturday. Determine the probability that both
were born on a Saturday.

1/2
1/4
1/7
1/13
1/14

The wording of the question was really to mislead us . Question is same as: Determine the probability that the second girl was born on a saturday given that first girl is born on a saturday. If 1/49 was among answer choices, then it could have maybe misled a few...
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Paul

CEO
Joined: 15 Aug 2003
Posts: 3454
Followers: 67

Kudos [?]: 874 [0], given: 781

Re: [# 22] 2 Min. Challenge : Birthday [#permalink]

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15 Mar 2004, 18:11
sorry guys, but you got to think harder
SVP
Joined: 30 Oct 2003
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Location: NewJersey USA
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15 Mar 2004, 21:20
CEO
Joined: 15 Aug 2003
Posts: 3454
Followers: 67

Kudos [?]: 874 [0], given: 781

Re: [# 22] 2 Min. Challenge : Birthday [#permalink]

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15 Mar 2004, 22:42
praetorian123 wrote:
Rules

1. Time yourself
2. Solve seperately

Two girls have their birthdays in the same Week. It is known that one
of the girls was born on a Saturday. Determine the probability that both
were born on a Saturday.

1/2
1/4
1/7
1/13
1/14

Well, i think this got to everyone. The answer is 1/13.

We know that at least one person has a birthday on Saturday. So for the probability that both were born on a Saturday, i first list the possibilities of birthdays for both girls .

G1-Girl 1
G2-Girl 2

G1 - G2
Sun-Sat
Mon-Sat
Tue-Sat
Wed-Sat
Thu-Sat
Fri-Sat
Sat-Sat
Sat-Sun
Sat-Mon
Sat-Tue
Sat-Wed
Sat-Thu
Sat-Fri

Note, we do not know which girl's birthday is on Saturday. So, we have to fix Each girls birthday on Saturday at a time and then list out the rest of the possibilities.

We have 13 possible combinations. Of those thirteen possible combinations that fulfill the condition, only one has two Saturdays, hence 1/13.
Founder
Joined: 04 Dec 2002
Posts: 14911
Location: United States (WA)
GMAT 1: 750 Q49 V42
GPA: 3.5
Followers: 3951

Kudos [?]: 25096 [0], given: 4746

Re: [# 22] 2 Min. Challenge : Birthday [#permalink]

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16 Mar 2004, 01:33
praetorian123 wrote:
praetorian123 wrote:
Rules

1. Time yourself
2. Solve seperately

Two girls have their birthdays in the same Week. It is known that one
of the girls was born on a Saturday. Determine the probability that both
were born on a Saturday.

1/2
1/4
1/7
1/13
1/14

Well, i think this got to everyone. The answer is 1/13.

We know that at least one person has a birthday on Saturday. So for the probability that both were born on a Saturday, i first list the possibilities of birthdays for both girls .

G1-Girl 1
G2-Girl 2

G1 - G2
Sun-Sat
Mon-Sat
Tue-Sat
Wed-Sat
Thu-Sat
Fri-Sat
Sat-Sat
Sat-Sun
Sat-Mon
Sat-Tue
Sat-Wed
Sat-Thu
Sat-Fri

Note, we do not know which girl's birthday is on Saturday. So, we have to fix Each girls birthday on Saturday at a time and then list out the rest of the possibilities.

We have 13 possible combinations. Of those thirteen possible combinations that fulfill the condition, only one has two Saturdays, hence 1/13.

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Manager
Joined: 28 Jan 2004
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Location: India
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16 Mar 2004, 02:30
Thats a really cool one.......
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16 Mar 2004, 06:04
simple yet beautiful. Kudos
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Best Regards,

Paul

16 Mar 2004, 06:04
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