$$25 : x :: x : \frac{81}{4}$$
$$\frac{25}{x}=\ \frac{x}{\frac{81}{4}}$$
$$\frac{25}{x}=\ \frac{4x}{81}$$
$$4x^2\ =\ 25\times81$$
$$x^2\ =\ 25\times\frac{81}{4}$$
$$x^2\ =\ 5^2\times\left(\frac{9}{2}\right)^2$$
$$x = 5\times4.5$$ [Here negative value is not possible. Because it is given that x > 0.]
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