Question 442530
The diagonal cuts the rectangle into two right triangles. Thus we can use the pythagorean theorem where our sides are the legs and the diagonal is the hypotenuse.

Let l be the length and let w be the width.

l^2 + w^2 = 10^2

Since the length is 2w, then we can can substitute l = 2w

{{{(2w)^2 + w^2 = 100}}}
{{{ 4w^2 + w^2 = 100}
{{{ 5w^2 = 100  =  w^2 = 20}}}

So our width is {{{sqrt(20)}}} or {{{2sqrt(5)}}}.