Okay, so her room is 30 sq ft right and her bed takes up 2/4. 2/4 is the same thing as 1/2 (half of her room.) Now, you need to find one half of 30 (30 / 2), that is 15. So if her bed takes up 15 sq ft, subtract that from the total area of the room. 30-15= 15 sq ft. This means that 15 sq ft of her room are left! :)
Hope that makes sense!