Mathematics 9709 · AS & A Level · Linear combinations of random variables

Linear combinations of random variables — practice question

A farm keeps $5$ ducks and $2$ geese. Four birds are selected at random. The random variable $X$ gives the number of geese selected.
(i)[3]

Construct the probability distribution for $X$.

(ii)[3]

Demonstrate that $\text{E}(X) = \tfrac{8}{7}$ and determine $\text{Var}(X)$.

(iii)[4]

When the farmer’s dog is released, it either chases the ducks with probability $\tfrac{3}{5}$ or chases the geese with probability $\tfrac{2}{5}$. If it chases the ducks, there is a probability of $\tfrac{1}{10}$ that they will attack the dog. If it chases the geese, there is a probability of $\tfrac{3}{4}$ that they will attack the dog. Given that the dog is not attacked, determine the probability that it was chasing the geese.

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