Barbara and Dianne go target shooting. Suppose that each of Barbara's shots hits the wooden duck target with probability $p_{1}$, while each shot of Dianne's hits it with probability $p_{2}$. Suppose that they shoot simultaneously at the same target. If the wooden duck is knocked over (indicating that it was hit), what is the probability that
(a) both shots hit the duck;
(b) Barbara's shot hit the duck?
What independence assumptions have you made?