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Tim throws a fair die twice and notes the number on each throw.

a) Tim calculates his final score as follows. If the number on the second throw is a 5 he multiplies the two numbers together, and if the number on the second throw is not a 5 he adds the two numbers together. Find the probability that his final score is

(i) 12,

(ii) 5.

b) Events $A$, $B$, $C$ are defined as follows.

A: the number on the second throw is 5
B: the sum of the numbers is 6
C: the product of the numbers is even

By calculation find which pairs, if any, of the events $A$, $B$ and $C$ are independent.

پاسخ تشریحی :
نمایش پاسخ

a)(i) $P$(final score is $12$) $ = P\left( {6,{\text{ }}6} \right) = 1/36$

(ii) $P\left[ {\left( {1,5} \right) + \left( {1,4} \right) + \left( {2,3} \right) + \left( {3,2} \right) + \left( {4,1} \right)} \right]$

$ = 5/36$

b) $P\left( A \right) = 1/6$

$P\left( B \right) = P\left[ {\left( {1,5} \right) + \left( {2,4} \right) + \left( {3,3} \right) + \left( {4,{\text{ }}2} \right) + \left( {5,1} \right)} \right]$

$ = 5/36$

$P\left( C \right) = 1 - P\left( {O,{\text{ }}O} \right) = 3/4$

$P$($A$ and $B$) $ = P$($1$ and $5$) $ = 1/36$

$ \ne P\left( A \right) \times P\left( B \right)$

$P$($A$ and $C$) $ = P\left[ {\left( {2,5} \right) + \left( {4,5} \right) + \left( {6,5} \right)} \right] = 3/36$

$ \ne P\left( B \right) \times P\left( C \right)$

None are independent.

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