two maths problems

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real_arsen_ac: Newbie | Next Rank: 10 Posts; Posts: 2; Joined: Wed Jul 23, 2008 6:21 am

by real_arsen_ac » Wed Jul 23, 2008 6:54 am

Hey guys, i got 2 problems here in OG:

1.The probability is 1/2 that a certain coin will turn up heads on any given toss. If the coin is to tossed 3 times, what is the probability that on at least one of the tosses the coin will turn up tails?
a.1/8 b.1/2 c.3/4 d.7/8 e.15/16
the answer is d

2.In a nationwide poll, N people were interviewed. If 1/4 of them answered yes to question 1, and of those, 1/3 answered yes to question 2, which of the following expressions represnets the number of people interviewed who did not answer yes to both questions?
a.N/7 b.6N/7 c.5N/12 d.7N/12 e.11N/12
the answer is e

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Source: Beat The GMAT — Problem Solving | Wed Jul 23, 2008 6:54 am

Canman: Senior | Next Rank: 100 Posts; Posts: 58; Joined: Fri Jul 11, 2008 5:00 am; Thanked: 4 times

by Canman » Wed Jul 23, 2008 7:28 am

Q1. You can solve this quickly by calculating the probability that heads will come up 3 times and understanding that P(E) = 1-P(not E).

So P(HHH) = (1/2)*(1/2)*(1/2) = 1/8
and P(not HHH) = 1-(1/8) or 7/8

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Canman: Senior | Next Rank: 100 Posts; Posts: 58; Joined: Fri Jul 11, 2008 5:00 am; Thanked: 4 times

by Canman » Wed Jul 23, 2008 7:29 am

I don't know where the smiley came from in my post

but last line should be 1 - 1/8 or 7/8

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CITI29: Senior | Next Rank: 100 Posts; Posts: 93; Joined: Tue Mar 18, 2008 9:28 am; Thanked: 2 times

by CITI29 » Wed Jul 23, 2008 9:09 pm

Ans for ques 2 shld be 'e'...is it?

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nitingupta5: Junior | Next Rank: 30 Posts; Posts: 21; Joined: Fri Jul 11, 2008 8:40 am; Thanked: 2 times

by nitingupta5 » Thu Jul 24, 2008 1:26 am

For the second question it should be

No of Yes for question 1 = N/4
No of Yes for question 2 = N/12(these are the people who said yes to question 1 also)

So there are supposed to be only N-N/12 people who said no to the question

Answer E

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