Question 47898
Not so easy, this one.
I shall assume you know your algebra, and do not require much explanation.
Otherwise, you have to feedback and I will edit my solution.
First, let us set up the problem.


Let w be the width and l be the length.
Then we have
(1): wl > 900
(2): 2w + 2l = 200


From the second equation, we get l = 100 - w,
which put into the first equation gives w(100-w) > 900
which expands out to {{{ 0 > w^2 - 100 + 900 }}}
which factorises to be {{{ 0 > (w-10)(w-90) }}}
This implies, hopefully you can see why, that 10 < w < 90.


However, because w < l , we also have that w < 50, 
from equation 2, or from l = 100-w.
Therefore, the answer is 10 < w < 50.