Let x liters be original amount of juice in container.
We have been given that Gavin drank 3/4 liters of juice from the container. We are also given that it is 3/8 of the original amount of juice.
Now let us set our given information in an equation.
[tex]\frac{3}{8}\cdot x=\frac{3}{4}[/tex]
After solving this equation we will get,
[tex]x=\frac{3\cdot 8}{3\cdot 4}\\ \\ x=\frac{8}{4}=2[/tex]
Therefore, initially there were 2 liters of orange juice in container.