Problem was slightly trickier than I originally thought...

Right let's look at this in terms on equations...

Break it down into 3 parts, the juice box (empty), the juice itself, and the grapes.

Now I'm assuming when you say half full juice box you mean a full sized box that is half full, not a juice box that is only half the size.

You have...

2 juice boxes + 2 juice = 18 grapes + 1 juice box + 1/2 juice on the first pan

On the second pan you have 1 juice box = 6 grapes...

So we can substitute that into the first pan to simplify things.

So I'll let you do that and if you need anymore hints I'll reply again.