There is a problem with this question. It has some information missing.
You would solve it on the following lines:
Let the unit cost of grape juice be
Let the unit cost of orange juice be
Let the unit cost of the blend be
Then we have
From this point on you would need to know the ratio of the cost of one type of juice to the other.
On the other hand; if they are the same then each quart for each type costs the same as
Just plucking a value out of the air:
I am only going to round off at the end to reduce rounding errors.
Using the determined value for