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A fair coin is tossed 5 times. What is the probability that

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Senior Manager
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A fair coin is tossed 5 times. What is the probability that [#permalink] New post 24 May 2006, 22:30
00:00
A
B
C
D
E

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A fair coin is tossed 5 times. What is the probability that it lands heads up at least twice?

a) 1/16
b) 5/16
c) 2/5
d) 13/16
e) 27/32

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SVP
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 [#permalink] New post 24 May 2006, 22:44
5/16

Two heads can be in a toss of 5 times in 5C2 = 10 ways

Each way(number of times at least two heads will come) has a probability of 1/32.

Hence 10*1/32 = 5/16
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 [#permalink] New post 24 May 2006, 22:45
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It is B....

Total possible outcomes=2*2*2*2*2=32
Favourable outcomes=5C2=10

Probability=10/32=5/16....
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 [#permalink] New post 24 May 2006, 22:52
SimaQ wrote:
It is B....

Total possible outcomes=2*2*2*2*2=32
Favourable outcomes=5C2=10

Probability=10/32=5/16....


going by your logic - the favourable outcomes should be 5C2 + 5C3 +5C4 + 5C5 ( question says at least 2..right?)
= 10 + 10 + 5 + 1 =26

hence prob = 26/32 = 13/16
ie choice D
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 [#permalink] New post 24 May 2006, 23:02
iced_tea wrote:
SimaQ wrote:
It is B....

Total possible outcomes=2*2*2*2*2=32
Favourable outcomes=5C2=10

Probability=10/32=5/16....


going by your logic - the favourable outcomes should be 5C2 + 5C3 +5C4 + 5C5 ( question says at least 2..right?)
= 10 + 10 + 5 + 1 =26

hence prob = 26/32 = 13/16
ie choice D


Yes.......
Calculated ofr only two heads.....

Favorable outcomes are 5C2+5C3 + 5C4+5C5
=> 10+10+5+1
=> 26
26/32 = 13/16

Hence D
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 [#permalink] New post 24 May 2006, 23:45
Yep my bad..... missed the at least part.... must be more concentrated.
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 [#permalink] New post 25 May 2006, 12:17
I don't understand whu you sum up "10+10+5+1".

Can someone explain it?
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 [#permalink] New post 25 May 2006, 22:31
netcaesar wrote:
I don't understand whu you sum up "10+10+5+1".

Can someone explain it?


We sum up the number of ways you can toss heads up 2 times, 3 times, 4 times and 5 times....
  [#permalink] 25 May 2006, 22:31
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