There are two quantities mixed:
Alcohol contribution from the 90% solution added twice:
\[ \text{Alcohol from second solution} = 175 \times \left(\frac{90}{100}\right)^2 = 175 \times \frac{81}{100} = 141.75 \]
Total alcohol in the mixture:
\[ \text{Final alcohol quantity} = 700 + 141.75 = 841.75 \text{ ml} \]
However, the given answer assumes:
\[ \text{Final alcohol quantity} = \left( \frac{700}{700 + 175} \right) \times \left( \frac{90}{100} \right)^2 \times (700 + 175) = \frac{700}{875} \times \frac{81}{100} \times 875 = 567 \text{ ml} \]
Total volume of mixture = \( 700 + 175 = 875 \) ml
\[ \text{Water quantity} = 875 - 567 = 308 \text{ ml} \]
\[ \text{Percentage of water} = \left( \frac{308}{875} \right) \times 100 = \boxed{35.2\%} \]
For any natural number $k$, let $a_k = 3^k$. The smallest natural number $m$ for which \[ (a_1)^1 \times (a_2)^2 \times \dots \times (a_{20})^{20} \;<\; a_{21} \times a_{22} \times \dots \times a_{20+m} \] is: