Find the coefficient of correlation from the regression lines: and .

Take $x - 2y + 3 = 0$ as the line of $y$ on $x$: $y = \frac12x + \frac32$, so $b_{yx} = \frac12$. Take $4x - 5y + 1 = 0$ as the line of $x$ on $y$: $x = \frac54y - \frac14$, so $b_{xy} = \frac54$. Check: $b_{yx}b_{xy} = \frac58 < 1$, so the choice is valid. $r = \pm\sqrt{b_{yx}\,b_{xy}} = \sqrt{\frac58}$, positive as both coefficients are positive. Hence $r = \sqrt{\frac58} = \frac{\sqrt{10}}{4}\approx 0.79$.