The size of a television screen is given as the length
of the screen’s diagonal. If the screens were flat, then
the area of a square 21-inch screen would be how
many square inches greater than the area of a square
19-inch screen?
(A) 2
(B) 4
(C) 16
(D) 38
(E) 40

Area of the square of 21 inch screen = L^2 where length of the square = diagonal/(2)^1/2 = (21/(2)^1/2)^2
similarly, area of the square of 19 inch screen = (19/(2)^1/2)^2

Difference = ((21)^2 - (19)^2)/2 = (40*2)/2 = 40

GMAT 2007