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# Probability Q

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Manager
Joined: 02 Aug 2007
Posts: 146
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Probability Q [#permalink]  15 Oct 2007, 11:32
Here's a probability question -

When tossed, a certain coin has the probability of landing on either side. If the coin is tossed 3 times, what is the probability that it will land on the same side each time?

A. 1/8
B. 1/4
C. 1/3
D. 3/8
E. 1/2

I chose A (1/2)(1/2)(1/2) = 1/8 (independent events) is this wrong?
Director
Joined: 22 Aug 2007
Posts: 572
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Kudos [?]: 14 [0], given: 0

Re: Probability Q [#permalink]  15 Oct 2007, 11:40
yuefei wrote:
Here's a probability question -

When tossed, a certain coin has the probability of landing on either side. If the coin is tossed 3 times, what is the probability that it will land on the same side each time?

A. 1/8
B. 1/4
C. 1/3
D. 3/8
E. 1/2

I chose A (1/2)(1/2)(1/2) = 1/8 (independent events) is this wrong?

I would choose 1/8, same reasoning...?
SVP
Joined: 01 May 2006
Posts: 1805
Followers: 8

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It's (B)

There are 2 faces that are different... So either u consider the first tossing meaningless or u have to consider 3 times one side then 3 times the other side.

P = 1 * 1/2 * 1/2 = 1/4
or
P = P(Side1) + P(Side2) = 1/2 * 1/2 * 1/2 + 1/2 * 1/2 * 1/2 = 1/4
Director
Joined: 22 Aug 2007
Posts: 572
Followers: 1

Kudos [?]: 14 [0], given: 0

Fig wrote:
It's (B)

There are 2 faces that are different... So either u consider the first tossing meaningless or u have to consider 3 times one side then 3 times the other side.

P = 1 * 1/2 * 1/2 = 1/4
or
P = P(Side1) + P(Side2) = 1/2 * 1/2 * 1/2 + 1/2 * 1/2 * 1/2 = 1/4

Got it! nice one

when tossed for the first time the probability that it will land on either side is one.

thanks, Fig.
Director
Joined: 18 Jul 2006
Posts: 531
Followers: 1

Kudos [?]: 30 [0], given: 0

(3C3 + 3C0)* (1/2)^3 = 1/4
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