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# What is the probability of getting no double "4" by rolling

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Director
Joined: 29 Oct 2004
Posts: 865
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What is the probability of getting no double "4" by rolling [#permalink]  21 Jan 2005, 14:15
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What is the probability of getting no double "4" by rolling two dices?

Which solution is correct:
p(no 4) * p(no 4)
5/6 * 5/6 = 25/36

or

p(no double 4) = 1 - p(double 4)
1 - 1/6 * 1/6 = 35/36

SVP
Joined: 03 Jan 2005
Posts: 2246
Followers: 13

Kudos [?]: 226 [0], given: 0

Second is right. The first one means no single 4 is allowed.
Director
Joined: 07 Jun 2004
Posts: 614
Location: PA
Followers: 3

Kudos [?]: 318 [0], given: 22

The answer to this will be 5/6 * 5/6 = 25 / 36

that is because they are independent events and in the first roll you want all the numbers execpt 4 so the P of occuring is 5/6

1/6 < 5/6 and the chances of 4 not occuring is more than 4 occuring

hope this helps
SVP
Joined: 03 Jan 2005
Posts: 2246
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Kudos [?]: 226 [0], given: 0

Was I wrong in understanding your question? I thought "no double 4" means you could have a four, as long as you don't get 4 for both rolls.
Director
Joined: 07 Jun 2004
Posts: 614
Location: PA
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Kudos [?]: 318 [0], given: 22

no double 4 means no no 4 AND 4 on the 2 die in a single roll
SVP
Joined: 03 Jan 2005
Posts: 2246
Followers: 13

Kudos [?]: 226 [0], given: 0

In other words you could have a 4 and a 5 in a two dice roll, for example. Your answer is still incorrect then.
Director
Joined: 31 Aug 2004
Posts: 610
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Kudos [?]: 50 [0], given: 0

25/36 is the case when you obtain a 4+X or X+4
SVP
Joined: 03 Jan 2005
Posts: 2246
Followers: 13

Kudos [?]: 226 [0], given: 0

Think about it this way. There are 36 different outcomes for a two dice roll. Only one outcome is excluded (the double four). The probability is 35/36.
Director
Joined: 29 Oct 2004
Posts: 865
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Kudos [?]: 68 [0], given: 0

Ops!! I had a 'brain fart' on this one.
Manager
Joined: 25 Oct 2004
Posts: 249
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Kudos [?]: 7 [0], given: 0

Good question ..............

Twixt could you explain what you mean by 4+x and X +4
VP
Joined: 18 Nov 2004
Posts: 1441
Followers: 2

Kudos [?]: 22 [0], given: 0

35/36 is correct.

1. 1st four and other not and vice versa = 2 * 5/36 = 10/36

2. No fours atall = 5/6*5/6 = 25/36

Total 35/36
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