I can't find the product moment coefficient of correlation? [closed]

These are the equations of least square regression lines:

$$Y = 20.8 - 0.219 X$$ ($$Y$$ on $$X$$)

$$X = 16.2 - 0.785 Y$$ ($$X$$ on $$Y$$)

Find the coefficient of correlation $$r$$.

closed as off-topic by Moishe Kohan, Leucippus, YuiTo Cheng, Shailesh, John OmielanMay 5 at 6:59

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Assuming you are talking about regression of one random variable on another (as opposed to finite sample approximations), the regression coefficient of $$Y$$ on $$X$$ is $$E(XY)/E(X^2)$$; that of $$X$$ on $$Y$$ is (by symmetry) $$E(XY)/E(Y^2)$$. So their product is $$E(XY)^2/(E(X^2)E(Y^2))$$.
A similar formula holds in the finite sample case, where you have expressions like $$\sum x_i y_i$$, and so on.