Hi,
The composite contribution margin is 26 that is because Plastic budgeted unit is 100,000 & Glass Frames budgeted units is 300,000 (total 400,000). I.e. the composite CM contains one Plastic frames unit and three Glass frames (one 5, three 7).
Hope that helps on question 28.
If you understand Q 28, 29 following the same logic.
For question 30, the composite contribution margin is 19 because one 5, two 7. (150,000 plastic, 300,000 Glass)
thus the break even units is (975000 /19)*3 = 153, 947 - choice C is the correct answer.
Hope that helps,
Sylvia