Let A denote the number of accidents a manual worker

4. Let A denote the number of accidents a manual worker in a particular factory has in a year. For a given worker A is modelled as a Poisson random variable with unknown rate parameter r that varies across the work force. Suppose that the rate parameter is modelled as a random variable R which has a Gamma distribution with shape parameter ?and scale parameter ?. Use the law of iterated expectations to show that the marginal distribution of A has a mean of ?/? and a variance of ?/? +?2.should get to a point where:E[A(squared)] = E[E[A(squared)|R]]= E[Var[A|R]+ (E[A|R])(squared)]= E[R + R(squared)]dont understand that part please explain please show full working out and and explain.