Lamar is an artist who has been commissioned to construct an ornate window. The window is to be in the form of an equilateral triangle surmounted on a rectangle, and the entire window is to have a perimeter of 20 feet. The rectangular part will be made of clear glass and will admit twice as much light as the stained glass triangular part. What dimensions should Lamar choose for the window if he wants it to admit the maximum amount of light?