When a certain biased die is rolled, a particular face occurs with probability $${1 \over 6} - x$$ and its opposite face occurs with probability $${1 \over 6} + x$$. All other faces occur with probability $${1 \over 6}$$. Note that opposite faces sum to 7 in any die. If 0 < x < $${1 \over 6}$$, and the probability of obtaining total sum = 7, when such a die is rolled twice, is $${13 \over 96}$$, then the value of x is :
A
$${1 \over 16}$$
B
$${1 \over 8}$$
C
$${1 \over 9}$$
D
$${1 \over 12}$$
Explanation
Probability of obtaining total sum 7 = probability of getting opposite faces.
Let A and B be independent events such that P(A) = p, P(B) = 2p. The largest value of p, for which P (exactly one of A, B occurs) = $${5 \over 9}$$, is :
A
$${1 \over 3}$$
B
$${2 \over 9}$$
C
$${4 \over 9}$$
D
$${5 \over 12}$$
Explanation
P (Exactly one of A or B)
$$ = P\left( {A \cap \overline B } \right) + \left( {\overline A \cap B} \right) = {5 \over 9}$$
$$ = P(A)P(\overline B ) + P(\overline A )P(B) = {5 \over 9}$$