Answer:
The sales mix is 1:2.
Model 101 Model 102
Selling Price 21 56
Variable Cost -14 -35
Contribution Per Unit 7 21
Multiply Sales Mix Ratio 1 2
Weighted Contribution 7 42
Now add the weighted Contribution to compute Contribution margin per composite unit which is 7+42=$49
Explanation:
I assumed that the cost and selling price here for Model 101 is $14 and $21 respectively. Similarly the cost and selling price of Model 102 is $35 and $56 respectively.
Remember that Contribution margin per composite unit means that we will earn 49 dollars(combined contribution of sales mix) if we sell the sale mix of Model 101 and 102 which is 1:2.