She can make 20 batches of pancakes with 4 gallons of milk.
Solution:
Given that,
Quantity of milk in refrigerator = 4 gallons
Quantity of milk required to make a batch of pan cake =

Need to determine how many batches of pan cake Mrs.White can make using 4 gallons of milk.
Number of batches which can be made by
gallons of Milk = 1
So number of batches which can be made by

So, number of batches which can be made by 4 gallons of milk = 4 multiplied by the number of batches which can be made by 1 gallons of milk =

Hence, she can make 20 batches of pancakes.