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Seyi
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If a coin is tossed 7 times, what is the probability that at 06 Dec 2004, 12:36
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If a coin is tossed 7 times, what is the probability that at least 3 out of 7 times it will show heads?



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If a coin is tossed 7 times, what is the probability that at 06 Dec 2004, 12:54
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1 - (1/2)^7 * (7C2 + 7C1 + 7C0)
= 1 - (1/2)^7 * (21+7+1)
= 1 - (1/2)^7 * (29)
= 1 - 29/128
= 99/128
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Paul
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If a coin is tossed 7 times, what is the probability that at 06 Dec 2004, 12:57
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venksune, the question said "at least" :)
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Seyi
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If a coin is tossed 7 times, what is the probability that at 06 Dec 2004, 13:40
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Thanks for the replies guys..

@ Paul....Did you do 3 sets of combinations because the question said "at least 3 out of 7 times" ?....

So if the question was
A coin is tossed 5 times, what is the probability that at least 2 out of 5 times it will show heads?

Then the answer would be

1 - 1/2 ^ 5 * ( 5C1 + 5C0) ?
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If a coin is tossed 7 times, what is the probability that at 06 Dec 2004, 14:25
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Seyi, that is right. I worked with unfavorable outcomes by doing total outcomes minus probability when you get 2H, 1H and 0H. Your example is also perfect
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venksune
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If a coin is tossed 7 times, what is the probability that at 06 Dec 2004, 20:55
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Aargh! Not again. 'atleast'. Agree with your working Paul. Thanks for pointing it out.
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jeremy02
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If a coin is tossed 7 times, what is the probability that at 08 Dec 2004, 10:13
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why can't you find out the probability of the coin coming up Tails 5 times out of 7 and subtract that from 1. can someone help explain this one to be?
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goostavo
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If a coin is tossed 7 times, what is the probability that at 11 Dec 2004, 03:32
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hi,

Would you please explain where the (1/2)^7 comes from? I'm having a difficult time figuring out which "n" or "r" to use in the combinations...

Thanks.
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If a coin is tossed 7 times, what is the probability that at 14 Dec 2004, 10:10
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jeremy02
why can't you find out the probability of the coin coming up Tails 5 times out of 7 and subtract that from 1. can someone help explain this one to be?
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This is a good reasoning but you also have to find the probability when tail comes up 6 times and 7 times because you want the unfavorable outcomes. Remember that question said "at least" 3 heads. This means outcomes when you have 5,6 and 7 tails or more simply put, outcomes when you have 0,1 and 2 heads.
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If a coin is tossed 7 times, what is the probability that at 14 Dec 2004, 10:15
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goostavo
hi,

Would you please explain where the (1/2)^7 comes from? I'm having a difficult time figuring out which "n" or "r" to use in the combinations...

Thanks.
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Prob of getting Heads: 1/2
Prob of getting 5 Heads: 1/2^5
Prob of getting Tail: 1/2
Prob of getting 2 Tails: 1/2^2
Prob of getting 5 Heads and 2 Tails: 1/2^5 * 1/2^2 = 1/2^7
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Paul
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If a coin is tossed 7 times, what is the probability that at 14 Dec 2004, 10:18
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Seth88
what is 5C1 and 7C1, etc? Sorry I'm new here
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nCr = 5C1 is read as 5 combination 1 and it is mathematically written as: n! / r!*(n-r)!
=5! / 1!(5-1)! = 5
I would suggest you to go through the probability guide at this link: https://www.gmatclub.com/phpbb/viewtopic.php?t=1585



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