PowerBI Total Cost Unit sums monthly calculations rather than creating a total for selected months

Question

I'm going to add a cost/unit metric to my dashboard. The issue is that if I pick more than one month, the monthly computations are simply added together. As an example,

Store	Month	Units	Cost	Cost/Unit
Blue	Jan	10	20	2
Blue	Feb	20	60	3
Red	Jan	10	30	3
Red	Feb	20	80	4

When I create a table with with these, and select January and February in the Splicer it shows something like this:

Store	Cost/Unit
Blue	5
Red	7

Of course this isn't the real total cost/unit for both months. Instead I'd like for it to show:

Store	Cost/Unit
Blue	2.67
Red	3.67

I'm new to PowerBI, so I apologise for the lack of words to express this, but any assistance would be greatly appreciated!