Set up a proportion:
1/7 L of water - 1/9 of the aquarium
x L of water - 1 aquarium (the whole aquarium)
[tex]\frac{\frac{1}{7}}{\frac{1}{9}}=\frac{x}{1} \\ \\
\frac{1}{7} \times 9=x \\ \\
\frac{9}{7}=x \\ \\
x=1 \frac{2}{7}[/tex]
1 2/7 liters of water are needed to fill the aquarium.
