1. The problem asks to find the ratio $n$ between the length of side $DE$ and the length of the corresponding side $AB$. 2. The ratio $n$ is defined as: $$n = \frac{\text{length of } DE}{\text{length of } AB}$$ 3. Given the ratio is $0.90$, this means: $$n = 0.90$$ 4. This indicates that side $DE$ is $90\%$ the length of side $AB$. 5. Therefore, the ratio $n$ between $DE$ and $AB$ is $0.90$. Final answer: $$n = 0.90$$