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# Probability - Heads

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Senior Manager
Joined: 10 Dec 2004
Posts: 273
Probability - Heads [#permalink]

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12 Jan 2005, 11:49
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Two coins are tossed with probability landing at Head not equal to 0.5. What is the probability of getting two Heads out of two tosses?

(1) The probability of getting head is 3 times that of getting tails.
(2) The probability of getting one Head and one Tail is 2/9
Manager
Joined: 06 Sep 2004
Posts: 69

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13 Jan 2005, 17:09
A.

P(head) = 3 P(tail)
Also P(head) + P(tail) = 1

So P(head) and P(tail) and therefore P(both heads) can be calculated.

B is insufficient. All it tells is P(A)*P(B)
Director
Joined: 07 Jun 2004
Posts: 612
Location: PA

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13 Jan 2005, 17:13
I would have to go with C as the Q is asking for the exact value

so we can get it from

a*b = 2/9

and a = 3b
Manager
Joined: 14 Dec 2004
Posts: 117

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13 Jan 2005, 18:06
Is this analysis correct?

We know that p(h)+p(t)=1
From condition 1: p(h)=3p(t)

Let x = p(t) where 0<x<1:

Then we have: 3x+x=1; Thus x=.25, and p(h)=3x=.75

Thus 1 is sufficient:

From 2, we can't get anything, so final answer is A
Director
Joined: 21 Sep 2004
Posts: 607

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13 Jan 2005, 18:06
i would go with A. explanation as above.
Director
Joined: 07 Jun 2004
Posts: 612
Location: PA

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13 Jan 2005, 18:18
guys the above calculation will work for 1 coin but 2 coins are tossed

What is the OA ?
Senior Manager
Joined: 22 Jun 2004
Posts: 392
Location: Bangalore, India
Re: Probability - Heads [#permalink]

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14 Jan 2005, 00:45
I would go with B.

P(Two coins with no heads) = 0.5

P(one head and one tail + all heads) = 1-0.5 = 0.5

(1)Insufficient for the above equation
(2)Sufficient because P(one head and one tail ) = 2/9
P(all heads) = P(one head and one tail + all heads) - P(one head and one tail ) = 1/2 - 2/9 = 5/18

Thus, it is B.

pb_india wrote:
Two coins are tossed with probability landing at Head not equal to 0.5. What is the probability of getting two Heads out of two tosses?

(1) The probability of getting head is 3 times that of getting tails.
(2) The probability of getting one Head and one Tail is 2/9

_________________

Awaiting response,

Thnx & Rgds,
Chandra

Senior Manager
Joined: 22 Jun 2004
Posts: 392
Location: Bangalore, India

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15 Jan 2005, 00:16
Could you justify the OA? Somehow, I could not!!

I appreciate if you let me know why I am wrong in choosing B. That helps me a lot.

pb_india wrote:
OA is "A"

_________________

Awaiting response,

Thnx & Rgds,
Chandra

15 Jan 2005, 00:16
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# Probability - Heads

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