make or buy

Hi John!

A company wants to decide whether to make its materials in-house or whether to sub-contract production to an external supplier. In the past it has made four materials in-house, but demand in the next year will exceed in-house production capacity of 8,000 units. All four materials are made on the same machines and require the same machine time per unit: machine time is the limiting production factor.
The following information is available.
Material W X Y Z
Units required 4,000 2,000 3,000 4,000
Variable cost of in-house
manufacture
$8 per unit $12 per unit $9 per unit $10 per unit
Directly attributable fixed
cost expenditure
$5,000 $8,000 $6,000 $7,000
Cost of external purchase $9 per unit $18 per unit $12 per unit $12 per unit
Directly attributable fixed costs are fixed cash expenditures that would be saved if production of the material in-house is stopped entirely.
If a decision is made solely on the basis of short-term cost considerations, what materials should the company purchase externally?

1. Th answer is 4,000 units of W and 4,000 units of Z.
2. Could you help me how to deal with the attributable FC?