Answer:
Width of the model is:
17.5 in
Explanation:
 The length of the classroom is 2.4 times its width. 
Length=2.4×width
 The length of the student's model is 42 in. 
Let w be the width of student's model.
By using the equation Length=2.4×width , we get
 42=2.4×w
⇒ 

⇒ w=17.5
Hence, width of the model is:
17.5 in