The answer is 4. I don't quite know how to show you my working, because I just got example numbers of the original dimensions of the room and found the area, and then doubled each and the area was always 4 times larger. You can think of it like because you're multiplying the width by 2 and the length by 2, you're doing 2 × 2 = 4. I hope this helps!
The ratio of boys to girls is 4:5. So let 4x be the number of boys and 5x be the number of girls. Then 4x+5x = 9x is the total number of students. Then the fraction of boys in the class is 4x/9x = 4/9.