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A cube with its sides numbered 1 through 6 is rolled twice

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A cube with its sides numbered 1 through 6 is rolled twice [#permalink] New post 28 Apr 2012, 06:33
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72% (02:45) correct 27% (00:54) wrong based on 2 sessions
A cube with its sides numbered 1 through 6 is rolled twice, first landing on a and then landing on b. If any roll of the cube yields an equal chance of landing on any of the numbers 1 through 6, what is the probability that a + b is prime?

(A) 0
(B) 1/12
(C) 5/12
(D) 7/18
(E) 4/9

I am not sure if the OA is correct. Thanks for help !
[Reveal] Spoiler: OA
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Re: Rolling primes [#permalink] New post 28 Apr 2012, 06:44
Cube has 6 surfaces like dice . So it could be taken as a dice.

Minimum value :1 Maximum value :6
So prime no's in the range : 1+1 and 6+6 = 1 and 12
prime no : 2 ,3 , 5, 7 ,11

Different combinations for getting these values :
2 1 1
3 1 2, 2 1
5 1 4 , 4 1 ,2 3 , 3 2
7 3 4 , 4 3 , 1 6, 6 1 , 2 5, 5 2
11 5 6 , 6 5

Probability = 15/36= 5/12

Hence C is the answer.
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Re: Rolling primes [#permalink] New post 28 Apr 2012, 06:48
raingary wrote:
Different combinations for getting these values :
2 1 1
3 1 2, 2 1
5 1 4 , 4 1 ,2 3 , 3 2
7 3 4 , 4 3 , 1 6, 6 1 , 2 5, 5 2
11 5 6 , 6 5

Probability = 15/36= 5/12

Hence C is the answer.


Ok, thanks! I was counting (1,1) twice ! I agree that its 5/12 (and not 4/9)
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Re: A cube with its sides numbered 1 through 6 is rolled twice [#permalink] New post 28 Apr 2012, 06:50
qtrip wrote:
A cube with its sides numbered 1 through 6 is rolled twice, first landing on a and then landing on b. If any roll of the cube yields an equal chance of landing on any of the numbers 1 through 6, what is the probability that a + b is prime?

(A) 0
(B) 1/12
(C) 5/12
(D) 7/18
(E) 4/9

I am not sure if the OA is correct. Thanks for help !


Total # of outcomes is 6*6=36;

Favorable outcomes:

a-b --> prime
1-1 --> 2;

1-2 --> 3;
2-1 --> 3;

1-4 --> 5;
4-1 --> 5;
2-3 --> 5;
3-2 --> 5;

1-6 --> 7;
6-1 --> 7;
2-5 --> 7;
5-2 --> 7;
3-4 --> 7;
4-3 --> 7;

6-5 --> 11;
5-6 --> 11.

Total of 15 favorable outcomes.

P=15/36.

Answer: C.
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Re: A cube with its sides numbered 1 through 6 is rolled twice   [#permalink] 28 Apr 2012, 06:50
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