Answer:
Heather's model has greater area by 24 sq.ft.
Explanation:
Given : Ellie's model represented 20 feet by 15 feet. 
 Heather's model represented 18 feet by 18 feet. 
To Find: Whose floor model represents the greater area? How much greater?
Solution:
Length of Ellie's model = 20 feet
Width of Ellie's model = 15 feet
Area = 

 = 

 = 

Length of Heather's model = 18 feet
Width of Heather's model = 18 feet
Area = 

 = 

 = 

Thus Heather's model has greater area by (324-300)= 24 sq.ft.