Conditional probability!

When two dice are rolled, what is the probability that the sum is 7, if it is known that either die shows 2?

7 36 \frac{7}{36} 11 36 \frac{11}{36} 2 11 \frac{2}{11} 5 11 \frac{5}{11}

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2 solutions

Jordan Cahn
Oct 11, 2018

There are 11 equally likely outcomes given that at least one of the dice shows a 2 2 : Die 1 Die 2 1 2 2 1 2 2 3 2 2 3 4 2 2 4 5 2 2 5 6 2 2 6 \begin{array}{cc} \text{Die 1} & \text{Die 2} \\ 1 & 2 \\ 2 & 1 \\ 2 & 2 \\ 3 & 2 \\ 2 & 3 \\ 4 & 2 \\ 2 & 4 \\ 5 & 2 \\ 2 & 5 \\ 6 & 2 \\ 2 & 6 \\ \end{array} Of these options, only two (2 & 5 and 5 & 2). Thus the answer is 2 11 \boxed{\frac{2}{11}} .

Parth Sankhe
Oct 15, 2018

Probability of a sum of 7 and a 2 on either, i.e. a 2 and a 5. = 2/36

Probability of a 2 on either = 1/6 +1/6 -1/36=11/36

Therefore, the answer is the ratio of these two = 2/11

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