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.