\[\boxed{\text{786\ (786).}\text{\ }\text{Еуроки\ -\ ДЗ\ без\ мороки}}\]
\[\textbf{а)}\ 5\sqrt{x} = 1\]
\[\sqrt{x} = \frac{1}{5}\]
\[x = \left( \frac{1}{5} \right)^{2}\]
\[x = \frac{1}{25}.\]
\[\textbf{б)}\ \sqrt{x - 4} = 15\]
\[x - 4 = 15^{2}\]
\[x - 4 = 225\]
\[x = 229.\]