Answer:
Step-by-step explanation:
The room's width should remain unchanged at 300 inches.
If the question is asking room of the same area but length shortened to 10 inches, then the answer is 400*300/`10 = 12000 inches.
One thing the question lacking (The area of both room is same).
[tex]\\ \rm\rightarrowtail 400(300)=390x[/tex]
[tex]\\ \rm\rightarrowtail 120000=390x[/tex]
[tex]\\ \rm\rightarrowtail x=307.69[/tex]