Using ratio and total juice
$450$ and $630$ are in the ratio $45:63 = 5:7$
$$
\begin{array}
\text{5 parts 42%} \\
\text{7 parts ?%}
\end{array}
\bigg\}
\text{12 parts 35%}$$
Since we are working in 'parts' and not specific units, we can simply multiply by 42 and 35, instead of finding 42% and 35%, because 42% and 35% are in the same ratio as 42 and 35.
Using fractions
450 and 630 are in the ratio 45:63 = 5:7, so the mixture is $\frac5{12}$ pineapple drink and $\frac7{12}$ orange drink.
So we know that 42% of $\frac5{12}$ of the mixture is juice and that 35% of the whole mixture is juice. Let $x$ be the fraction of the orange drink that is juice. Then:
$$\begin{align}
\frac{42}{100}\times\frac5{12}\hspace{6mm}+\hspace{5mm}x\times\frac{7}{12}&=\hspace{3mm}\frac{35}{100}&\\
\Rightarrow \frac{6\times7}{20\times5}\times\frac{5}{6\times2}\hspace{1mm}+\hspace{5mm}x\times\frac7{12}&=\frac{7\times5}{20\times5}\\
\Rightarrow \frac7{20\times2}\hspace{8mm}+\hspace{5mm}x\times\frac7{12}&=\hspace{3mm}\frac{7}{20}\\
\Rightarrow x\times\frac7{12}&=\frac{7\times2}{20\times2}-\frac7{20\times2}\\
\Rightarrow x\times\frac7{6\times2}&=\frac{7}{20\times2}\\
\Rightarrow x\times\frac16&=\frac1{20}\\
\Rightarrow x&=\frac6{20}\\
\Rightarrow x&=\frac3{10}
\end{align}$$ Which is the same as 30%.