Probability Dice
1. **State the problem:**
We roll a fair dice twice.
Event R: sum of outcomes is 7.
Event S: product of outcomes is 12.
Find $P(R)$, $P(S)$, $P(R \cap S)$ and determine if R and S are independent.
2. **Total sample space:**
Since each dice has 6 sides, total outcomes are $6 \times 6 = 36$.
3. **Event R: sum is 7.**
Possible pairs $(x,y)$ with $x+y=7$ are:
$(1,6), (2,5), (3,4), (4,3), (5,2), (6,1)$.
Number of such outcomes = 6.
So, $P(R) = \frac{6}{36} = \frac{1}{6}$.
4. **Event S: product is 12.**
Possible pairs $(x,y)$ such that $x \times y = 12$ and $x,y \in \{1,...,6\}$:
$(2,6), (3,4), (4,3), (6,2)$.
Number of such outcomes = 4.
So, $P(S) = \frac{4}{36} = \frac{1}{9}$.
5. **Intersection $R \cap S$: sum is 7 and product is 12.**
Common pairs from both sets are:
Event R: $(1,6), (2,5), (3,4), (4,3), (5,2), (6,1)$
Event S: $(2,6), (3,4), (4,3), (6,2)$
Intersection: $(3,4), (4,3)$
Number of such outcomes = 2.
So, $P(R \cap S) = \frac{2}{36} = \frac{1}{18}$.
6. **Check independence:**
Events R and S are independent if and only if
$$P(R \cap S) = P(R) \times P(S)$$
Calculate the right side:
$$P(R) \times P(S) = \frac{1}{6} \times \frac{1}{9} = \frac{1}{54}$$
Since
$$P(R \cap S) = \frac{1}{18} \neq \frac{1}{54} = P(R) \times P(S)$$
Therefore, R and S are **not independent**.
**Final answers:**
$$P(R) = \frac{1}{6}, \quad P(S) = \frac{1}{9}, \quad P(R \cap S) = \frac{1}{18}$$
Events R and S are not independent.