Sandwich : Juice = 2 : 3. Sandwich is 20% of money, so find juice cost as a percentage.
\\(\begin{aligned}
\frac{\text{Sandwich}}{\text{Juice}} &= \frac{2}{3}
\\
\text{Juice} &= \frac{3}{2} \times \text{Sandwich} = \frac{3}{2} \times 20\% = 30\%
\\
\text{Total spent} &= 20\% + 30\% = 50\%
\\
\text{Money left} &= 100\% - 50\% = 50\%
\end{aligned}\\)
Final Answer: 50%