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# What is the probability of getting a six on either the first

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Intern
Joined: 28 Nov 2005
Posts: 31
What is the probability of getting a six on either the first  [#permalink]

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29 Sep 2007, 03:37
What is the probability of getting a six on either the first or second toss of a normal six-sided die ?
A) 1/36
B) 1/18
C) 1/6
D) 11/36
E) 1/3

I've got 2 methods to solve this, but I don't get the same result with the 2.
Thanks.

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VP
Joined: 08 Jun 2005
Posts: 1136

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29 Sep 2007, 03:42
1
getting six on dice roll ---> 1/6

5/6*1/6 ---> second toss
1/6*5/6 ---> first toss
1/6*1/6 ---> both

5/36+5/36+1/36 = 11/36

Intern
Joined: 28 Nov 2005
Posts: 31

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29 Sep 2007, 08:06
Thanks for you quick reply KillerSquirrel !

My mistake came from forgetting to take into account the "BOTH" as belonging to either 1st OR 2nd ?
VP
Joined: 28 Dec 2005
Posts: 1462

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29 Sep 2007, 08:18
wow i was totally off the ball.

To me, I just thought of it like this:

prob of 6 on roll 1 = 1/6

prob of 6 on roll 2 = 1/6

therefore, prob or 6 on 1st or 2nd roll = 1/6 + 1/6 = 1/3

Where did my logic go wrong ?
VP
Joined: 08 Jun 2005
Posts: 1136

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29 Sep 2007, 08:24
1
pmenon wrote:
wow i was totally off the ball.

To me, I just thought of it like this:

prob of 6 on roll 1 = 1/6

prob of 6 on roll 2 = 1/6

therefore, prob or 6 on 1st or 2nd roll = 1/6 + 1/6 = 1/3

Where did my logic go wrong ?

Since you have one outcome where you get 6:6 then you need to remove this duplicate of six's (you counted it once in the first roll then in the second roll) hence 1/6+1/6-1/36 = 6/36+6/36-1/36 = 11/36.

VP
Joined: 28 Dec 2005
Posts: 1462

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29 Sep 2007, 08:37
Ah i think i see it now:

we have to subtract the probability that we get 6s on BOTH rolls, which would be 1/6*1/6=1/36

So , if the answer were just 1/6 + 1/6, how would the question need to be worded differently ?
Intern
Joined: 28 Nov 2005
Posts: 31

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29 Sep 2007, 09:05
pmenon wrote:
Ah i think i see it now:

we have to subtract the probability that we get 6s on BOTH rolls, which would be 1/6*1/6=1/36

So , if the answer were just 1/6 + 1/6, how would the question need to be worded differently ?

I guess it would be something like : "What is the probability to guet 6 ONLY at the first or ONLY at the second roll ?"
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Re: What is the probability of getting a six on either the first  [#permalink]

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29 Nov 2017, 01:41
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Re: What is the probability of getting a six on either the first &nbs [#permalink] 29 Nov 2017, 01:41
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# What is the probability of getting a six on either the first

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