Answer:
A. 15 cups
B. Diagram below
We are given that,
The ratio of dressing to the vegetables is 2 cups : 4 pounds.
A. It is given that the vegetables are of 30 pounds.
Let the number of cups be x.
Since, the ratio of dressings and vegetables will remain same.
We have,

i.e. 
i.e. x= 15
Thus, we will need 15 cups of dressings for 30 pounds of vegetables.
B. As we have obtained the fractions,
2 cups : 4 pounds and 15 cups : 30 pounds
Using the butterfly method, we see from the diagram that these fractions are equivalent because the product of the numbers is equal to 60.