Let X denote the proportion of employees at a large firm who will choose to be covered under the firm’s medical plan, and let Y denote the proportion who will choose to be covered under both the firm’s medical and dental plans.
Suppose that for 0 \leq y \leq x \leq 1, X and Y have the joint cumulative distribution function
F(x,y) = y(x2 + xy - y2)
Calculate the expected proportion of employees who will choose to be covered under both plans.