Answer:
3.5 inches on the scaled model
Explanation:
So using the ratio, 1:90 I thought of two ways to solve this
1) Proportion 
2) Equation
Using the proportion method
1:90=x:315
Now cross multiply 
315=90x
Now solve and you’ll get 3.5
Equation is just 
315/90=X?
Just solve and you’ll also get 3.5