Answer:
60 percent
Explanation:
Contribution margin refers to the revenue a firm derives after deducting the variable cost it has incurred.
Contribution margin = Sales - Variable costs
Contribution margin or contribution to sales ratio represents the percentage of contribution a firm earns from the sale of it's output.
It is represented mathematically as,
= 
Also, contribution margin ratio = 100 - variable cost ratio percentage.
Hence, contribution margin for three departments would be:
A = 100 - 30% = 70%
B = 100 - 40% = 60%
C = 100- 50% = 50%
This represents if sales revenue is 100, contribution margin earned is 70, 60 and 50 under three cases.
Since sales revenue in all three departments is the same, let us assume the sales revenue of a department as y.
Thus, weighted average contribution margin would be, 60 percent