Question 857594
That makes an important-to-know special triangle.  The remaining angle is 90 degrees, which you would expect, knowing that the diagonal is in a rectangle.  Each triangle formed is half of an equilateral triangle.  The diagonal of your rectangle is one of the equilateral triangles equal sides.  The short leg of the special triangle is HALF the diagonal, or 10 inches.  


The long leg is the same as the long side of the rectangle.  Find it using Pythagorean Theorem for Right Triangle:
{{{y^2+10^2=20^2}}}
{{{y^2=400-100=300}}}
{{{y=sqrt(300)}}}
{{{highlight_green(y=10*sqrt(3))}}}-----the long side of the rectangle.
Remember, short leg, the shorter side of the rectangle, is 10.


I leave the perimeter part to you.