My professor explained this concept absolutely horribly and I have no idea how to do these problems. Let A and B be independent Poisson random variables with parameters α and β, respectively. Find the conditional expectation of A given A + B = c. (Hint: For discrete random variables, there is no conditional density. Use the definition of conditional probability.) Attempt: Starting with the definition, f(A | A + B = c) = [f(A, A+B=c)] / [f(A+B=c)] Not sure how to proceed.