Drawing balls without replacement

A bag contains balls numbered with integers from $$1$$ to $$7$$ (inclusive). Two balls are drawn from the bag without replacement, having values $$x,y.$$ The expected value of $$xy$$ can be expressed as $$\frac{a}{b}$$ where $$a$$ and $$b$$ are coprime positive integers. What is the value of $$a + b?$$

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