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fmeinsen
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Five coins are tossed one after the other. What is the 05 Nov 2006, 10:33
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Five coins are tossed one after the other. What is the probability that the first three are heads?

Any input appreciated that explains how I could get to the right answer.



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SimaQ
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Five coins are tossed one after the other. What is the 05 Nov 2006, 10:55
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The probability that the first 3 are heads is: 1/2*1/2*1/2=1/8

The probablity that the first three are not heads: 1-1/8=7/8....
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fmeinsen
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Five coins are tossed one after the other. What is the 05 Nov 2006, 12:25
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Nope, answer is 5/16
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joeydvivre
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Five coins are tossed one after the other. What is the 05 Nov 2006, 13:40
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Nope, the answer is 1/8.
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Sumithra
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Five coins are tossed one after the other. What is the 05 Nov 2006, 13:56
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I get
Probability that three are heads = 5/16
Probability that first three are heads = 1/8
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trivikram
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Five coins are tossed one after the other. What is the 05 Nov 2006, 14:00
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5C3*(1/2)^3*(1/2)^2 = 5/16
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fmeinsen
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Five coins are tossed one after the other. What is the 05 Nov 2006, 14:07
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Hey trivikram

is there a difference between the folloing two questions?

1) Five coins are tossed one after the other. What is the probability that the first three are heads?

2) Five coins are tossed one after the other. What is the probability that overall three are heads?

Do both result in 5/16?

Thanks
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joeydvivre
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Five coins are tossed one after the other. What is the 05 Nov 2006, 14:46
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Of course there's a difference.

1) ANS = 1/8
2) ANS = 5C3 (1/32) = 10/32 = 5/16
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fmeinsen
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Five coins are tossed one after the other. What is the 05 Nov 2006, 15:26
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According to the GMAT Club Question collector the answer to question1 is
5C3*(1/2)^3*(1/2)^2 = 5/16
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trivikram
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Five coins are tossed one after the other. What is the 06 Nov 2006, 05:30
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joeydvivre
Of course there's a difference.

1) ANS = 1/8
2) ANS = 5C3 (1/32) = 10/32 = 5/16
Show more


I think this is correct...

the 1st one the event are independent of the total # of coins

but the latter one has a sample of 5 and we got to decide on that
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joeydvivre
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Five coins are tossed one after the other. What is the 06 Nov 2006, 05:41
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Yeah, but you can still be fooled on these.

For example -

1) I choose a card from a normal deck of cards, observe it is a heart, and then choose another card. What is the probability it is a club?

2) I choose two cards from a deck and throw them away. Then I choose a third card, what is the prob. it is a club?
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fmeinsen
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Five coins are tossed one after the other. What is the 06 Nov 2006, 17:18
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My solution would be:

1) Assume card is not being replaced & Assume that a card deck represents 52 cards: (52/4)/(52-1)

2) We do not know what type of cards the 1st two cards were..
a) assuming they were clubs: ((52/4)-2)/(52-2)
b) assuming they were not clubs: ((52/4))/(52-2)



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Hi there,
This topic has been closed and archived due to inactivity or violation of community quality standards. No more replies are possible here.
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