Crazy problem solving question..

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reach.ran: Junior | Next Rank: 30 Posts; Posts: 11; Joined: Wed Feb 27, 2008 4:52 pm

by reach.ran » Sun Apr 27, 2008 9:28 am

Can someone please explain me how to derive this answer.. please.. thanks..

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Source: Beat The GMAT — Problem Solving | Sun Apr 27, 2008 9:28 am

freedsl: Junior | Next Rank: 30 Posts; Posts: 25; Joined: Wed Mar 26, 2008 10:30 am; Location: so cal

by freedsl » Sun Apr 27, 2008 10:24 am

they are asking for bread only buyers which equals neiter( C or M) buyers which equals 1-probability of M&C ONLY buyers.

C only = 50-20 =30
M only = 40-20 =20

then P (M&C) = 20+30/100 or 1/2

1-1/2 =1/2 probability of M only.

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reach.ran: Junior | Next Rank: 30 Posts; Posts: 11; Joined: Wed Feb 27, 2008 4:52 pm

by reach.ran » Sun Apr 27, 2008 10:52 am

freedsl wrote:they are asking for bread only buyers which equals neiter( C or M) buyers which equals 1-probability of M&C ONLY buyers.

C only = 50-20 =30
M only = 40-20 =20

then P (M&C) = 20+30/100 or 1/2

1-1/2 =1/2 probability of M only.

Thanks for answereing, but the correct answer is 3/10 not 1/2. thanks again.. someone else please help me..

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freedsl: Junior | Next Rank: 30 Posts; Posts: 25; Joined: Wed Mar 26, 2008 10:30 am; Location: so cal

by freedsl » Sun Apr 27, 2008 12:12 pm

What i did wrong was take 20 off twice, anyway

So you have the formula ALL buyers that trade in M and C (Unique buyers) = M+C- both (M&C) = 50+40-20 =70 the prob for this is 70/100 or 7/10

the opossite which would be only B buyers is 1- 7/10 =3/10

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jscoligan: Junior | Next Rank: 30 Posts; Posts: 12; Joined: Mon Jan 21, 2008 11:12 pm

Kaplan formula

by jscoligan » Mon Apr 28, 2008 3:24 pm

Cake+muffin+neither-both=Real total

50+40+neither-20=100

neither=30

30/100=> 3/10

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Stuart@KaplanGMAT: GMAT Instructor; Posts: 3225; Joined: Tue Jan 08, 2008 2:40 pmFollowed by: 614 members; Location: Toronto; Thanked: 1710 times; GMAT Score:800

Re: Kaplan formula

by Stuart@KaplanGMAT » Mon Apr 28, 2008 7:43 pm

jscoligan wrote:Cake+muffin+neither-both=Real total

50+40+neither-20=100

neither=30

30/100=> 3/10

Kaching!

Perfect question for the overlapping sets formula:

True # of items = # with characteristic 1 + # with char 2 + # with neither char - # with both chars

In this case:

100 = 50 + 40 + neither - 20
100 = 70 + neither
30 = neither

So, probability of finding a "neither" person is 30/100 = 3/10

Stuart Kovinsky | Kaplan GMAT Faculty | Toronto

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