Answer: Buy Zed since the relevant cost to make it is $60.
Explanation:
Cost to produce Zed to Beta is $80.
If Beta buys Zed from a supplier, they will still incur 40% of their overhead which is:
= 40% * 50
= $20
Added to the cost of buying, total cost if Beta buys Zed would be:
= 58 + 20
= $78
This is less than the cost to produce so Beta should buy Zed from the supplier.