The formula for the expected value of a function g(x,y) of two continuous random variables X and Y is given by:
$$E[g(X,Y)] = \iint_{-\infty}^{\infty} g(x,y) f(x,y) \,dx\,dy$$
In this case, $g(x,y) = xy$, and the joint PDF is given by:
$$f(x,y) = \frac{1}{5}(x+2y)
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