\(\left(5^{103}-5^{102}-5^{101}\right):\left(5^{99}.26-5^{99}\right)\)
\(=5^{101}.\left(5^2-5-1\right):5^{99}.\left(26-1\right)\)
\(=5^{101}.\left(25-5-1\right):5^{99}.25\)
\(=5^{101}.19:5^{99}.5^2\)
\(=5^{101}.19:5^{99+2}\)
\(=\frac{5^{101}.19}{5^{101}}\)
\(=19\)