Students often ask me how we can acquire so much information from the seemingly little given in the problem. As with all geometry word problems, it helps to draw a picture. The other thing to notice immediately is that the computer monitor is "perfectly square". This is not an accident!
We can use what we know about 45-45-90 triangles to solve for the side lengths, which allows us to solve for the area of the whole monitor, which allows us to solve for the area of the screen.
Two tiny pieces of information (square, 20 inch diagonal) really open up the door!
Thoughts on the answer?