To simplify the expression, rewrite all terms as powers of 5: $25 = 5^2$ and $125 = 5^3$. This transforms the expression into $\frac{5^x \times (5^2)^{x-1}}{(5^3)^{x+1}} = \frac{5^x \times 5^{2x-2}}{5^{3x+3}}$. Combining the exponents in the numerator and simplifying gives $5^{x + 2x - 2 - 3x - 3} = 5^{-5}$.