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Re: Peter rolls two dice at the same time. What is the probabili [#permalink]

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29 Sep 2015, 21:18

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Peter rolls two dice at the same time. What is the probability that he will roll a double?

a) 1/12

b) 1/36

c) 1/6

d) 5/12

e) 1/3

Notice that the probability doesn't change if we roll one die first and the other die second. So, let's do this.

Let die #1 the first die, and let die #2 be the second dice

P(both dice are the same) = P(die #1 is any value AND die #2 matches die #1) = P(die #1 is any value) x P(die #2 matches die #1) = 1 x 1/6 = 1/6 = C

ASIDE: Some people might wonder how I found the first probability to equal 1. We need to recognize that the first die can be any value and, in order to get a double, the second guy must match the first die. Once we have rolled the first die, there is a probability of 1/6 that the second die matches the first die.